If time and money isn't an issue, most anything can be put in there with some fabrication and welding time.... That 401 with the right cam and a good working carb makes a ton of torque. The engine that comes to mind that would be a bolt in is the Buick 425 Nailhead, if you can find one.... I saw a '64 last summer that had one of the newer 455 Buick GS engines in it, it was very well done and to the non Buick knowledgeable guy looked like a factory install..... But it also looked like it had been a bunch of work, including some mods to the oil pan....