REMCO is best known for its Lube Pumps, to make automatic transmissions/transaxles towable when the otherwise were not.
They also offered Driveshaft Disconnects. What I started out wanting to say was call REMCO Tech Support and see if anybody can talk with you about this manual transmission.
Problem is, REMCO spun that DD business off to somebody else, and I don't know if the expertise followed the product, stayed at REMCO or simply "Left the Building." Here's FAQ #6 from REMCO's site
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Do you make the Drive Shaft Coupling (Disconnect) that I can use to tow my Rear-Wheel (RWD) or Four-Wheel Drive (4WD) vehicle?
No. Remco sold that product line in 2008 and we no longer are involved in the manufacturing or sales of that product. If you are interested in a Drive Shaft Coupling (Disconnect) please contact either Drive Shaft King in Dallas, TX at 800-442-6329 or Just for Towing in Yuma, AZ at 928-782-3611.
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I'd still like to know if the transmission in your prospective Mustang, was used in other vehicles, and if those vehicles were considered flat towable.