I'd do an updated version of dave's super duper mileage 318.

1) start with a mag 318. I'd pull the heads, measure how down in the bore the pistons are. should be ~.055 or so.

tear down to bare block, have the block decked ~0.035 to 0.040" and then run the Mr. Gasket .028" thick head gaskets

get the cam reground by bullet, using bullet's HR259/316 lobe for intake and exhaust. more economy, drop to the HR252/320 lobe on the intake

do a little home porting on the magnum heads following dulcich's may 2005 mopar muscle article-- get the heads redone, opening up the valve job to take advantage of the 1.92/1.625 valves. use hughes #1110 springs and retainers. redrill for LA intake

intake, I'd look for an LD340 or LD4B, open up to accept a t-quad, run a small tquad

I'd run headers.

tranny, I'd keep it simple and run an iron case A833OD if I could find one. aisin R154 would be second choice.

rear gear, 3.23's with the A833OD is a nice combo with 25-27" tall tires. if running 27-28" tall tires, you could go to a 3.55.