408 build

The 1.88 valved heads is good for about 375 hp if opened up to 2.02 standards 400 hp if you do a full port job 450 hp plus.

1.88 valves and some port work could get you 400. There's a sticky on porting stock valves 318 heads you should check out, 318willrun has a good video on the same thing and look up some of the Steve Dulcich 400 hp 318 and 360 recipes on the Web they have some info on porting if your up to trying.

One problem you'll have is your short block is probably gonna be set up for 10:1 CR and quench with closed chamber heads.
So your cr will be a little low for the cam you'll want to run some milling could help but since it's temporary I'd just take a bit off and live with it.