Jim, a few points you should consider:
1) there are indeed two types of heater cores - the smaller "factory-A/C" one and the larger "hi-output" one. If your truck left the factory with an A/C system installed, whether it is currently operational or not, you should use the "factory-A/C" type heater core. The "hi-output" core was installed only in trucks that had no A/C from the factory, it uses a different plenum box from a factory-A/C truck and sits kinda where the evaporator core is in a factory-A/C truck, so you physically cannot fit the non-A/C core in your truck.
2) the core itself in these trucks does not care how you install the hoses on it, as both inlet and outlet pipes are the same size (5/8") and there shouldn't be any flow restrictors built in either of them. If by any chance there is a flow restrictor in your new core (never see one with it, but anything is possible), the pipe it is in should be clearly marked as "inlet" and the restrictor itself should be visible. Again, never seen such a thing right out the box.
3) the need for restrictors is actually caused by the flow capability (or lack thereof) of the heater core, in many vehicles the water pump when spun at high rpms will push out more coolant than the core can handle, leading to "ballooning" and eventually rupturing the core. In that sense it is not the "static" coolant system pressure (16lbs for gassers usually, 13lbs or less for IDIs) that causes it, it's the "dynamic" pressure from the heater core dead-heading the incoming coolant flow. I have personally done this with a bull-nose Chevy truck, as a result of a heavy-duty pump and my practice of shifting at 6000 rpms - it killed the an aftermarket heater core first, and then a dealership-bought one. I know of several Fox-body owners who had the same experience, only it didn't take them 22 minutes from start to finish to replace their cores like I did. Ford actually recognized this as a problem at some point, and certain vehicles in the late-'80s and early-'90s did get a flow restrictor from the factory - I don't recall which models exactly those were (heavy CRS condition going on here), but I do remember the restrictor is a sort of a bushing that slides in the "inlet" heater hose right before said hose slips onto the heater core pipe. This is actually not the best solution, as now the dead-head is moved from the core to the hose itself, but I suppose it's cheap enough and replacing a heater hose is usually a whole lot easier than replacing the entire heater core. To be honest tho it is also what I did to my Chevy truck as well, only I used a flanged nut of suitable size (1/2" IIRC, threads drilled out and edges chamfered for smoother thru-flow) instead of a Ford-style bushing and slid it into the heater core's inlet pipe till the flanged end of the nut was firmly against the edge of the pipe, this way the dead-head pressure from the coolant could not force the nut inside the heater core, and the nut being steel would not deform under it either. Never blew a core again, and never noticed reduced heat output either, Mustang buddies copied it onto their cars as well with no ill effects that I'm aware of.
4) flow restrictors in IDIs - yes they do exist, only not in the bushing-in-the-hose form like some gassers got - instead Ford (or IHC, probably IHC) got smart and instead of using your typical wide-open 5/8" thru-hole barb fitting on the passenger side engine head where the heater core coolant supply comes from, they installed a barb fitting that has the outer diameter of size appropriate for 5/8" hose, but its inner diameter (where coolant flows through) is less than that, if memory serves me correct the thru-holes is actually about 1/2" in diameter. Why this is the smart and IMHO proper way of restricting heater core flow - the dead-head pressure is now created in the engine head coolant passage, and the engine head really couldn't care any less about it, resulting in low-pressure heater core AND supply hose, even if you're revving up against the governor. So, because this flow restriction is built into your engine, and not in the heater core itself, theoretically it shouldn't matter which way you hook up your supply and return hoses on the heater core - each of the pipes can be either inlet or outlet, core shouldn't care one bit. IIRC the typical installation involves designating the pipe closer to the engine as an inlet and the one closer to the fender as an outlet because it makes for neater factory hose routing, but if you have a coolant bypass filter hanging off your fuel filter tower you may find out that having the outboard pipe as inlet and the inboard one as outlet may work out better for your hose routing.
Hope this helps, bottom line is buy a core for a "factory-A/C-equipped" truck and I'd recommend that while you're at it you replace your heater hoses as well, this way you know you're good to go for years to come.
*a quick foot-note - considering the factory fitting that goes in the head is as per Gary's info no longer available, my solution would be using a radiator lower-tank fitting from an automatic-transmission-equipped truck - this thing is very similar to what lives on the engine head, probably the same NPT as well (3/8" IIRC), but its thru-hole is very small, like 3/16" small - measure your in-head fitting's orifice diameter, drill the rad-mount fitting to that size, and install it in the head, and because the rad-mount fitting does not have a rubber sleeve pressed into it like the factory in-head fitting it theoretically will not deteriorate with time either. I dunno why the factory chose the rubber-sleeve route for the in-head fitting, but I do not see any disadvantage to using the drilled out solid-brass fitting. Gary, are you aware if these are still available from Ford? If they are not they could be sourced from salvage yards, and being what they are they should not have suffered any significant deterioration that would render them unusable for the purpose described above...
