I swapped 4 speed transmissions on my 52 1 ton (for another SM420).
No problem removing the old one, and somehow managed to sit on the seat and get the second one in place by myself.
... then found out I couldn't install the clutch fork with the transmission in place....
Removed transmission, install clutch fork, and got some help putting the transmission back in.
That was no fun but all it cost was a little time.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-