Atf for 904/999 transmission

I have been using the mopar atf+4 in my 999 torqueflite. It is all original only thing I did was take it apart clean the valve body and install a shift kit. Im running a boss hogg converter. Im about to replace my transmission pan gasket for the 4th time at 8 dollars a quart atf4 is getting really expensive can I use the cheaper dextron stuff or should i stick to atf4?