60 or 70 years ago a lot of guys used to hook two transmissions together to get more power to pull very heavy loads and that worked well when you turn the rear trans around to make an over drive the main problem you had was the input shaft was designed to be held centered by the clutch and pilot bearing so if you didn't make an extra bearing suport for it the bearings in the center of the transmission were short lived. so you will need to pick a trans with a large enough input shaft to be able to machine it for a support bearing or build an intermediate shaft that will accept the clutch spline on one end and the driveshaft yoke on the other and mount it with two heavy duty high speed pillow block bearings with lock collars to prevent the shaft from moving forward and back. The new shaft must be mounted in perfect alignment with the output shaft, and so that if the transmission moves under load the shaft will stay in line. When you over drive a transmission you are creating continous loads that the old trans was not designed for. Once you was under way and shifted to high gear it was a direct drive with no loads on the gears but when you start to over drive you will be running in second gear on the back transmission mile after mile. This was the problem that had to be solved with modern O D transmissions and that is why many tell you not to tow a trailer in OD
The intermediate shaft can be made from an old output shaft for the rear splines then Guys made the connection for the trans input shaft by machining a tube and that slid over the end of the output shaft and allowed the input shaft to slide inside then they took a clutch hub and set it up in a lathe turned a small shoulder to hold it square in the end of the tube and welded it and the intermediate shaft to the tube with multiple pass welds then chucked it back up and machined the weld just enough to make it even so they were near balance.
Sorry for the long post but I hope this helps.
