Hi,
Nice find, 401 as you know requires special 65/66 GS only engine mounts and exhaust manifolds, if you got them it's a fun combo
Also trans has to be a nailhead pattern, different from a BOP
300 4v is a nice performer as well, not bad on mpg for long distance travel
The st300 on either will be a limited performance due to the 1.76? first gear, a th400 2.48 or a th350 2.52 and 200r4 2.72? will have better off the line performance.
The 300/355 might work nice with a 200r4, one from a GN, Monte SS or 442 as better guys and power levels closer to what the trans will hold, otherwise they are pricey to rebuild, I have one getting built for a Olds 455 and it's in the 3 grand price range.
You could always do a stick conversion, all fun even in a 300 car, have a 4 door 4 speed with 3.23 fun combo
Best of luck with the project
Regards,