need build advice

Here's the deal; Pick one; 15 mpg or in the 12s. Trying to do both with a small block is an exercise in frustration.
Going 400 plus cubes gets you a better chance, but the trend is to to go overkill right out of the gate and gone is economy.
Like who builds a 12 second car and cares about mileage.

You wanna go 12s its about $2000, just about any old 360-4spd will get you there with a 4bbl and 4.56s
You want 15mpg its about $200. Any old teener-904 with 2.76s will do it with a decent tune-up.
You want both, you need about $15000Plus US bucks.
4000 for the engine,1000 for support works,3500 for the 5-speed, 2500 for a rear that hooks with the right gears and tires,2000 for suspension,steering,BRAKES, and front tires, 1000 for exhaust,and throw in 1000 for miscellaneous.
I think that's even not enough.