I priced a 4 barrel intake for the /6 and it's cheaper to get one for a V/8. I don't know how much pistons would be. Dollar for Dollar the 318 would be the best way to go
You have gone on and on about wanting to do a 318 over a 340/360 but that's ok but a 273 is just one step to much over the line lol. The OP thinking about doing a 273 makes more sense then your obsession with a super torquey low rpm 318.