318, 340, or 360?

Basically the bigger the engine the less gear and stall and better idle for a certain hp.

Also depends if your using just the block or block/crank or block/crank/heads.

If you just using block engines will be within 20 cid or so. Not the biggest deal.

Block/crank all 3 same power potential but 318 over 360 will need more gear stall and have a ruffer idle for the hp level.

If using stock heads will limit hp to

273/318 = 300hp ish
340/360 1.88 =375hp ish
2.02 and magnum = 400hp ish

Porting and or aftermarket sky is the limit

Saying all that 360/5.9 is probably your best starting point even if you wanted super mild.
Stock magnum with manifolds and 4bbl makes about 300 gross hp.