Replacing the heater-core WILL fix the window-fogging/poor-heat problem; however, it WILL NOT fix the pressure problem.
The heater-hose system would simply by-pass, even when a cut-off valve completely closes off the flow.
Does it lose any coolant ??
I will relate my mysterious coolant situation that continues EVEN AFTER AN ENTIRELY DIFFERENT ENGINE SWAP.
For years, with the original IDI 6.9, I could pull up to the pumps, shut the engine OFF, fuel the tanks, and then REMOVE the radiator-cap with no fear of losing a drop of coolant.
Then, just out of the blue, no matter even if the engine had sat cold and un-used for a week, or had been driven hard twenty minutes before, when I would loosen the cap, coolant under pressure would spray all over the place.
It was never low; and, the only coolant ever lost was that that sprayed from under the cap when loosened.
I tried and tested everything and never did figure out the cause.
I tried probably half-a-dozen replacement caps to no avail.
Then, I swapped the I-H IDI-6.9 for a 6BT Cummins; and, much to my surprise, still had the spewing radiator cap situation.
Still never a drop of coolant lost, no over-heating issues, no other problems whatsoever.
I ran the old original radiator for a few months after the Cummins swap; then, just because it was old and possibly ready to cause problems, I installed a brand-new radiator; still no change in the spewing problem.
I replaced the plain cap with one that has a pressure-release lever.
So long as I always lift the lever first, I can remove the cap anytime I please and no more pressure spewing coolant all over.