Years ago while in the process of building a 401 race engine, I read somewhere that Mopar 440 sixpack valves would fit in the 401-390 heads. They are larger than AMC vales, much stronger and flow much better. I had them installed in a set of early 390 dog leg heads. Also I had the heads machined for screw in studs. I cut Ford Boss 302 guide plates in half and used those to guide the push rods along with roller rockers. Very strong engine.
I believe a picture of that set up is shown here in the forum. They are in a 74 beige AMX.