Greg H,
The transmission cover and hole are different. On the cover is the pivot for the gas pedal.
If you searched this forum, you found many comments about differences between manual and HydraMatic trucks. Here is one excerpt:
"Be sure you get the shift linkage, push button from the dash (to engage the special starter), starter with solenoid, frame crossbar (to support the rear of the transmission), the accelerator pedal and transmission cover for the cab floor. Also, be sure to get the shift indicator that goes on the steering column (it is cute and it is rare) and check to see if the mounting holes for the indicater are different than for a 3-speed (if they are, get the steering column). The brake master cylinder is unique to the1/2 ton Hydramatic and my Factory Assemblty Manual shows that the brake lines are routed differently."
Also, Mechainiac once posted this link as a possible source of parts:
http://www.autotran.us/hmparts1.html Here is another possible source: http://www.bright.net/~nrthwest/
Good luck to you. Please keep us informed of the progress of your project. As soon as I find a Hydramatic frame and cab (pipedream?), I will take on the same project (I have a drivetrain and all associated parts).
Tim
