Kevin, I would suggest the MT75. Well trod path with good support from Spyder. The frame change is the ideal time to take this on. A few details....
I sourced the rebuilt transmission with the required linkage modifications directly from Spyder. Note they have since modified the linkage to reduce sizzle. My shift lever sits slightly aft of stock, but I find it better positioned than stock.They can probably supply the correct clutch for your BDA. The MT75 uses an axial style internal slave cylinder, and Spyder will supply a flexible hydraulic line to connect directly to your existing master cylinder. They also have the corrct length prop shaft to match to your Plus 2 diff, starter motor, conversion fitting for the Plus 2 speedometer cable, and a transmission mount that matches the MT75 casing. For some reason my package did not include the sheet metal sandwich plate pieces to cover the bottom of the bell housing, but Spyder may have something available there and just forgot to supply. If all of the details are too much to take on, I am sure Spyder would attach your engine to the new gearbox ready for installation.
Note the MT75 is best fitted to a replacement Spyder space frame. As you are doing the frame anyway this is the right time. Make sure Spyder fully understand you are fitting the MT75, as they modify one of the members next to the starter to ease fitment issues. You will also want to confirm clearance for the harmonic balancer at the frame vacuum tank/crossmember. I am not familiar with the BDA details, but Spyder have different crossmembers for the TC and their Zetec builds. The best is to fit everything into the new built up subframe prior to re-installing the body.
As mentioned, the preferred MT75?s are from the Scorpio. The Scorpio enthusiast?s site has the complete service manual posted. The parts drawings are posted, but unfortunately not the corresponding Ford part numbers. This becomes confusing as there were several distinct versions of the MT75 and a few running changes. My local shop had to make up a few relatively simple special tools to rebuild the box. Even though my tech is very experienced, he found the box service quirky; not a immediate issue if you purchase a rebuilt box from Spyder.
Two important details from my build. Shorten the Spyder shift lever and have a threaded connection made. This allows removal of the lever so the engine and transmission can be removed as a unit with the body in place. Note the threaded bellhousing holes are all metric threads; although your SAE bolts from the four speed will fit they will destroy the metric threads; you don?t have to ask how I know.
There are some drawbacks to the MT75. The gearbox was not used in North America so we have parts sourcing issues over here, not an issue in the UK though. The first gear is somewhat lower than ideal; the rest of the gearset is pretty well spaced and the 0.82 overdrive is great. Routing the exhaust around the bellhousing is a bit difficult and compromised; I have taken a second try at mine with a new big bore exhaust and have my fingers crossed regarding ground clearance.
HTH
Stu