Wilcap mfg makes a nice adapto-kit for that one. I just used one to put a tremec 3550, ( very similar to a T-5),behind an 8BA flatmotor in a 48 C.O.E. The part was top quality as were the throwout bearing adapter and hardware. you will have to make up a new rear mount adapter, move and modify the stock crossmember, or use a universal crossmember. The shifter in the cabover had to be relocated because tremec shifter was too far back. I think that one of the mustang places like Mustangs plus can help with that if you need it. Looky here: http://www.mustangsplus.com/Merchant...Code=new_items
You will need to get a clutch disc made special because the flathead flywheel does not have the depressed center to clear the damper springs like the later stuff does. We had Superior clutch in San Jose Calif make it up for us. The pilot bushing and the stock pressure plate worked ok but we still had to turn a slight relief in the flywheel to clear the hub in the clutch disc. It took about 10 minutes on a 16" lathe. here's the website for wilcap: http://www.wilcap.com/
The owner, Pat I think is his name, is really helpful. If you need more info I can take a picture or 2 of the job I did and post it here. It was alot of work but I know it sure made that old cabover a joy to drive. Hope this helps.