Shaft seals & the actual gasket for the two piece torus could be leaking. They were known for that.
Check all the simple stuff first.
a later model turbo-hydramatic 3 speed transmission can be mated to your engine using an adapter plate that can be had through WILCAP
Contact them for pricing and instructions.
If you absolutely have to ave an original 4 speed hydro there are parts rebuilding services for the old hydramatic 4 speeds.
Web search like dogpile will bring up a bunch of resources.
Here are a few results.
Unless you are married to the old 4spd Hydramatic for nostalgia reasons swap it out.
The later model dual coupling hydramatics were prone to shredding the fluid coupling if high power abuse was applied.
They were not as rugged as the earlier hydramatic trans that all the old gassers used.
GM ceased production by 65/66
They were gone from racing by the early 1970's