I used to try to run the 302 casting high swirl heads on my MPG small blocks, but then one time tried the 308 milled to closed on a zero deck 318 motor and MPG jumped up quite a bit, on another 440 for a friend we swapped on some eddelbrocks in place of the 516 and mpg jumped up. A third was a 99 durango (I know EFI...) we put on a set of RT heads and his 5.9 and mpg jumped up a lot so I gave up on the small port theory (since then I have found the magnum is way happier and efficent with big ports especially in the pushrod pinch RT RHS Eddy...). I think when the piston starts sucking through a very small well designed valve curtain area (sharp valve edges, no shrouding...) the fuel is va-poo-rized and atomized as it's sucked through there very fast then as well as the high vacuum lowers boiling point of the hydrocarbons so what's happening in the port is not that important as long as it's not a flow restriction. I used to argue with my grandpa for years about how he was wrong because of port velocity when he would tell me they would maximum port a cylinder head and get much better MPG (1970s), till I started trying it myself after he was gone and found out the old man wasn't so dumb after all. I think it's one of those things that is great in theory but not reality. The only thing it (very small ports) really does is act like a governor. Use the cam timing and compression to keep vacuum very high instead of a restricted port size.

Heck even a 906, 452, 915 BB head is barely if any any better than a magnum or X head. Those 413 motor home heads are way too small.

I would seriously get a lock up converter, especially if you make a very high tq motor in a motor home with a gear vendor, non lock up is gonna generate a TON of heat in the trans and lose a lot of efficiency, then you also don't have to compromise on the stall speed you need to get that big rig moving. They are not un-reliable, I have re-built hundreds of those trannys (727, 904, a-500, 518 and REs) with lock-up and never seen any big issues caused by lock up.

As for compression, you could easily add another point maybe two if you get everything else just right with premium fuel. It seriously helps MPG, TQ, HP... I did a 318 with flat tops and magnum heads milled .030 and .029 piston to head clearance and it helped that little motor dramatically in every way (99 1/2 ton long bed ex cab with a slide in camper). That was the only mod, even put the cam, old rings and bearings back in after zero decking the block to get a true comparison.