1HP per cubic inch is as easy as picking up the phone, and emptying your wallet
An 'HONEST' 350HP 350 is going to take some work. Remember the 70 Z28 350 had a 360HP 350,,, To make that they used 11:1 compression, the 186 casting heads that flowed an honest 225cfm @ 0.50, a mechanical lifter cam to make the RPMs to get to 360HP,,, and recomended 96 octane.
In short that was the BEST of the muscle car era that gave a daily drivable 350HP smallblock. Today (and I do not say this to insult anyone) you will find about 99% of the smallblocks you hear about that make "about 400HP" are really making more like 325 as installed, with all the accessories and 'real' conditions.
Today things are much easier, technology has gave us a lot,, BUT.... if your wanting to spend $1500 on a stock 350-, and expecting a honest 350HP, and 15MPG.. I think your going to fall somewhat short of the goal.
To do that in a heavy chassis like a 72 LWB,, your asking a LOT!!!