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?
I had the same question on a 904 and after researching the web, found out type F works and you can blend it with ATF 3/4 to get crisper or smoother shifts. More ATF 3 = smooth, more Type F = crisp.