3.9 Stroker
The main problem with 6 cylinders is you got 6 intake and exhaust ports, 75% of the power capability of a similar V8 head plus smaller displace gonna need more rpm to make power. A 375 hp 6 cyl is a similar build to a 500 hp 318/390. 300-400 hp is a decent range for a 318, which is 225-300 hp for a 3.9l. Not saying don't build it but don't expect much from it. Unless turbo is a possibility.