Slant six turbo cost analysis: 300 hp

The stock super six manifold people treat a little more like gold than it is. You LIKELY won't find one for under $150.00 unless you found it in a junkyard yourself.

Also blowthrough mods shouldn't cost $500 if being done by yourself.
Nitrophyl float would be about $20.00 and making a power valve is free if you have a bench grinder and some JB weld. Hacking the choke tower off is free or can be done with a $7.00 hacksaw, and then after that it's just drilling a rejetting.