Interesting (unusual) transmission advice

I checked the fluid level in Park while hot, level surface. The first time I checked it was clearly over a lot. I siphoned off a quart. Didn't make much difference. I add 12oz of Auto trans treatment, not much difference. 

I checked one of the receipts sent to me by the PO, it was a $1200 bill to a trans place. I called them, got this older guy and right away he was edgy, thinking I was a complainer. I just asked him if he could recall the work and if he might have an idea where to start with the issue using reverse.  I said the first time I checked the level it was high and he said "Oh yes, those FMX transmission have to be overfilled. Ford told us that at the seminar in 1974". I said really? They didn't recall or issue a proper dipstick? "No. That's what they told us, I been working on transmissions since I was 14.....etc. "

Well, I thanked him for his time.   Really confused now, I added another 12 OZ of treatment and 1/2 qt of Type F. Drove the car a few miles, not any better, not much different. I'm going to siphon off the extra and get the level back on the FULL mark, then just live with it until the swap. I am guessing there's an issue that fluid level or treatment won't do anything for.

Ditto, a real shop is best to possibly help with that, or a shift kit company familiar with that trans. Except for experts, you might be stuck living with it or taking a shot at a shift kit(which are super scarce for an FMX). There is very likely a VB solenoid which directly controls and affects the reverse; you might find that and be able to R&R it to inspect it or more. So are you ready to pull the valve body and dig into it?

All of the old 3-speeds are good choices for a nice mild build, but most people are aiming at some upgrade. The C4/C6/FMX are great for a basic mild build old Ford.