Yeah as long as it's correct for three speed, chances a four speed toploader tail housing fits a three speed are zero & none.
I did some searching and tail housings for the three speeds are few and far between. There was one for an early Mustang on ebay for a reasonable price but it had a totally different mounting pad for the transmission mount. I agree on the four speed, doubt very much they would interchange.
When I bought my '70 the previous owner had installed one of those universal shifters with the big a$$ mounting bracket like in Craig's picture. (Still have the 3-speed and shifter in my shed 47 years later) Since the '70s tunnel is smaller than later years he had cut the support and used a BFH on the side of the tunnel for clearance. The bracket made the shifter sit farther front than a factory installed shifter.
I've heard they're interchangeable to a point. As I recall the 4 speed will work on a 3 speed, but not the otherway around. He can call David Kee and find out
Not sure where you landed with this but if you're up for a drive to Eastern Washington this would solve all your issues! https://www.facebook.com/marketplace/item/3199599376729587/
