I like straight six's. (in Jeeps)
Clifford Performance specializes in performance parts for straight six engines. They are pricey though.
The Company name is 6=8
Why not build the internals, then as your budget allows add headers, cast aluminum intake, carb ect?
Headers from Summit
There is a stroker kit for the 258, but it is a budget buster.
If you look in my sig, I still kind of miss my CJ7. Never should have sold it. It had a very solid motor with cam, headers, intake and carb, and had work done to the head to make it flow better. It was a while ago but I think it had valves from something else. It had gobs of power and happily revved to 5000 rpms.
I think you can use the better flowing head from the 4.0.
Look for part that are emission approved.
Do lots of research and make a plan and a list. Build it right the first time and it will serve you for a few hundred thousand miles.