The '70 LT-1 motors made that power with 11:1 compression (forged pistons), a solid lifter cam and 2.02/1.60 valves on straight plug (non-angle) cylinder heads. I know...I had one in a nearly bone stock '70 Z-28 and it ran 13.70's @ 106 in the qtr mile.
You could easily make that same power with 9.5:1 compression, a hydraulic cam that is ground on a better profile, and aftermarket aluminum heads that flow so much better. Heck, even the aftermarket IRON heads are better than what they offered in '70. Throw in a high energy ignition (MSD) and an Edelbrock Air-Gap intake with a decent 750cfm
Holleyesque four-barrel and 400HP is very reachable.
I'd help you on cost, but I've been away from the SB chevy game too long to tell you otherwise. Go to
www.hotrod.com and they probably have some good SB chevy forums over there.
-Dean