390 vs 408

It may be easier in your case to stroke the 318,with an overbore its not far off a 4in. bore and you already have the block.The main benefit of the 4in. arm is torque anyway and you will not lose anything significant going from a 3.940 to a 4.030 bore size.You can make an average mopar head flow extremely well with a chevy 1.94 intake-or for that matter a 1.88,it really comes down to whats easiest for you to obtain and cost.Either engine built similarly would run similarly-not apples and oranges.