Common stroke is 3.85 done with a stock crank, offset ground and .030 bore makes 426 inches, using a 401 block and crank. the 390 is shorter deck but only .033. Ro-DY will do crank as can shaft tech. use 6 inch small chevy rods at 2 or 2.1 and then custom pistons. For what you are doing it may be too much expense for 20 cubes as the stock crank and rods will do fine.

My car is 437 inches, indy heads.