I can confirm the 85-91 square body clutch master also works with the newer internal slave cylinders in the NV3500/NV4500 transmissions as well.
I used a Swagelok tube fitting to connect the metal tubing off the stock 85 clutch hose to the plastic tubing of the clutch internal slave. I never had any problems with leakage. Expensive fittings but they do not loosen with vibration and have zero leakage up to over 10,000 PSI. I have witnessed them hold onto tubing without any leakage until the stainless steel tubing burst at 35,000 PSI and be vibrated violently for years without coming loose.
The tubing sizes are not the same, so you'll need to order a reducing union of the proper sizes ( 1/4" to 3/16" iirc)