Motor Swap on a 63

I guess the 65 Valiant must be different cuz for me, in 1973 or so, a 340 dropped right into the hole, that the slanty had just vacated.
I sure wouldn't let a lil firewall get in the way of a V8 swap. But a 273 wouldn't be my V8 of choice, either.
Huh, that's about 46 or more years ago....... criminy I'm getting old.