all 1 GARY's pricing and real complexity is way over inflated.
Just don't get 1gary to build it. Most work for the US DOD.
Google search for a MSD 5462 box on ebay. as a start.
Then search the basic SBC long WP 142 satin blower KIt. Its $2000
summit Blower cam and lifters $99.
Used holley 750 $75 -150. Proform HP body Kit $125. Drill bit to drill carb body for BR powervalve $5
Headers any bolt on long tube header is good.
hummm good 305 stock motor as a donor Humm FREE to $100 max.
Rings, bearings, gasket rebuild set to freshin it up. Hum $85
complete new 500hp capable fuel system with Carter electric pump and 3/8" lines and a filter $125-150.
Does not even need a regulator. keep it simple.
Complete cylinder head rework (you port them) under $500
Pretty basic. Keep it simple.