1978 318

Don't spend $$$ putting 1.88 1.60 valves into 273 heads or any of that crap unless you want to do a lot of porting to make them work right. It's easier to work with the stock valve size if you're going to go that route ...aka the 200-210cfm intake flow range.
If you want to stay with a 318 in that truck and not stroke it to 390 cubic inches then you'll need to get more air into it and out of it.
Ultimately a set of heads will help... but the cid falls short of big low rpm grunt