Recommendations for mail order transmission?

The OP needs to re-think his task and rebuild the actual transmission that is in the car now.

Then you know what you got. Some of the used cores are basket cases in rely tuff shape.

Like our government lying to everyone to hide the truth, those selling you the "cores" will say most anything to get them sold and turn them into money.

Have to remember the good stock is pretty much used up. Reminds you of craigslist rule: Don't believe a word until the person selling has proven trustworthy.

Screenshot_20200828-220947_Gallery.jpg
Fresh Built 904 pictured here.

Edit:
Besides putting in a transmission out of a different car, makes your car no longer a numbers matching car.