These trannys were built on ATF3, ATF+4 wont hurt, but its usually more expensive, so why pay extra? ATF3 probably is going to be very hard to find also, as all the new stuff runs +4... You can mix them, no problem. Just dont mix atf3 and dextron without an additive. .
Just dont use straight Dextron, but you can fill with the cheaper dextron 3, and use a product called LubeGuard in the black (not red) bottle to modify the friction of the fluid. Probably should use 2 bottles on our trannies. . its a bit expensive, but also contains lube enhancers to renew old seals etc. . (which you dont have to worry about methinks)
good luck
-j