The reason I suggested lowering the pump itself was to help overcome the lift or "suction head" that gear pumps are not good for...they can push like heck, but pulling a draw is problematic, until well primed and in your case, well heated fluid. The dilemma with those bronze gear pumps is wear. They really wear the end plates creating a loss in pressure. Also, even softer chunkies can mar the teeth causing yet another point of wear/loss of pumping ability.
The tank access plates are really easy to install. I sell pre-fabbed access plates with gaskets, bolts and plates. Basically, choose a hole size, use a hole cutter to size, drill holes for bolts, install plates. The neat thing about having a plate, besides tank access for cleaning and inspection is, you can install lotsa stuff on them, fuel supply lines, return lines, gauges, senders... whatever you need. If you eff it up, get another plate! No worries about mis-locating anything in the main structure of the tank. I have them in all my customers boats for easy inspection and some modifications.
Don't forget about the heating pads for filters. They're stuck on the outside, with less volume than the tank and will harden up faster. You could use one of those as your resistor, very small amperage draw...when you need the big tank heated, use a 110V house system like your block heater. Needs bigger power to heat anyway, this way you don't over-stress your 12V electrical system. 200gal is a lot of heating in 40 degree weather....imagine lower temps! Oh wait... you already have a 12V hot water heater element installed in the tank? Why not put two or three in there, plus the filter heaters.
When you "upgrade" your alternator, you should be going to a single wire hook up. That one wire will go to either your isolator or directly to one of the 12V batteries.
Before spending all that money on batteries and wiring, get the alternator. This way you will have the power at hand when you start putting things together.
Wire size = total amps x length plus 10% times 2... I like to make sure there's plenty of copper in my runs... prevents lotsa over-heating. I figure in 10% extra just for additional working room. I like to leave 3-4" at each end for cutting out corroded wire down the road. Boats are corrosion havens. Ground directly to the frame of the truck under the bed. Your tank should be grounded too, to prevent static discharge, especially on reacted fuel that may have too much alcohol left in it from processing... or motor oil that may have been victim to fuel dilution....you never know sometimes. Fuse the hot link too. Those pumps will freeze up on you from time to time, gotta knock start 'em like a bad truck starter. This will cause a spike in power... don't want it to fry wires.
