The best thing to think about is if you have a 4" bore, 4" stroke V8 alongside a same bore/stroke I-4 or V6, the V8 will make more power due to the CI inherently. You'd do more modifying the I-4 or V6 to compare with the V8 and then some, so if you are already prepping it for RWD, why wouldn't you just swap in the V8. It is definitely the cheapest route to go to make power.
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'67 Dart 270 4 door '72 340(9.5:1), 727, 8-3/4", A/C, original interior, big bolt discs
'77 Aspen SE 2 door '77 318, 998,8-1/4",A/C, Cruise control car,power discs,power steering