1991 4l V6 has blown the freeze plug behind the passenger side motor mount...

[Turdle]

Does the engine run well? Try reading codes for unknown problems. If it checks good, I'd just fix the freeze plug and drive it.

 

[410Fortune]

check the condition of the transmission fluid, is it bright red and no burn smell? If so then keeping it cool is your best buddy. Consider adding a large aux transmission cooler

I would fix the freeze plug and go from there.......... let her earn the next round of parts

 

[BKennedy]

I would support the engine and replace that freeze plug. The engine does not have to come out. If you pull the engine mounts, you can jack the engine up about two inches. Mechanic needs a new mechanic. Pulling the engine on a 32 year old car can cause you a lot more trouble.

After replacing the freeeze plug, change all the fluids. Then put some miles on it and see what else needs to be addressed.

If your overall plan is to do an engine swap as suggested, then don't spend a minute on it until you have gathered all the parts and are ready to rip the entire drive train out.

 

[410Fortune]

also if one freeze plug is leaking then consider changing them all on that side, then repeat on other side.
The others are likely not far behind
I use some scotch brite to clean the block then alcohol to wipe free of oil and then add an anaerobic sealant (never hardens) like flange sealant from
Loctite around the edges, finally careful press them in with a socket or the old plug