I really need some advice here

I agree, build the '91 360. The 308 heads are pretty capable with a little work, I have a set on the 340 in my Duster. I had the valves opened up to 2.02/1.60 and a stage 2 port done on them, they flow 264 cfm @ .500". You can definitely get more flow with aftermarket heads, but you can still make a lot of power flowing 264 cfm. The only thing is that if you're paying someone to do the porting and machine work, it may actually be cheaper to buy the aftermarket heads. If you're doing most of the work yourself, the 308's are a great way to go.