how much transmission fluid do I need?

That transmission is not going to hold up. You need to seriously consider replacing that transmission BEFORE it comes apart, as your converter will then be junk.

The 700 only STARTED to get good in 1987 when the casting was changed to include the auxillary accumulator (whatever it was, the whole trans casting changed). The 6.2L diesel not only made no power relatively, they also shifted at an EXTREMELY low RPM.

Trust me. I had to pay $1500 to replace the trans in my 1983 K5, which was a 700R4, and I learned all about the garbage trans the 700 was until the very last years of its use, before it went "E" (4L60 is also a 700, 4L60E has no TV cable and requires a computer to run correctly).

The 700 was NEVER designed to handle that much power. Look at any Camaro or Corvette with the 700 from 1982-1986 and see how much power they were making. You will note that the fastest GM car of that era (the Buick GN) did NOT use the 700R4, it used the 2004R.

That number does include draining the torque converter. If you just pull the pan and replace the filter, it only takes about 4 quarts at most. This amount is approximate and the level should be checked before driving; engine/transmission warm and running in Park. Automatics are fragile in nature and break all the time. To put a number on how much it can handle is futile because it will eventually fail anyway. Most automatics can not pull a trailer and be used in overdrive at the same time (weak). If you want durability, get a stick shift and HD clutch. Putting a high stall torque converter on a low rPM diesel is counter-productive. They are designed for built gas engines (read high rpm) that don't idle well below 1,500.

How about calling a GMC/Chevy dealer and asking them?