Welcome Corey,
Think about the Subaru H6 boxer engine too mate - they go hard and sound silky smooth. 6 speed manual too
Years ago in Street Machine I read about a 4 cylinder conversion from a Datsun 200B into an FC; I think the theory was they had similar HP outputs but the Datto engine was more economical; sadly something that we need to consider these days.
You'll get heaps of input from members on here who have done the 304 V8 thing (NES, Craig, Pete to name a few).
There's some quirky stuff too; a 2J project on here which looks awesome, and Ben (GrandmasterB on here) is doing a 1UZ V8 which will be great (if I were to do it again I'd follow a similar path). Pete on here (who has a 304 FC ute) is also doing an RB26DETT Nissan GTR powered FC Holden which is smooth as silk.
Josh