2nd and 3rd have syncroes on the 3 speed. A 48 4 speed should be a sm420 and it should be syncroed in 2, 3, and 4. No it will not bolt up to your 1941 bellhousing. The 1948 bellhousing will bolt up to your 1941 engine, and your 1941 frame... unfortunately it will not bolt up to your brake and clutchpedal assembly. I believe Jim Carter sells a modified bellhousing that will accept the later transmissions and the earlier pedals. Another option is to use an adapter plate and a t5 5 speed. The 41 3/4 ton should have an open drive rearend from the factory, so all that would be required would be to remove the front closed driveline and have a 1 piece driveshaft made. Aditionally a rear transmission tail support may or may not be needed. At the very least, I would modify the carrier bearing crossmember to cllear or attach to the t5.
There is plenty of info on the t5 swap if you use the search feature. There is a nice tech tip on installing one in an art deco truck.
John
P.S. on my 1940 gmc, I am running a well built 302. I decided to use a nv4500 transmission with a homemade adapter plate. I didn't think a t5 would hold up to the type of loads I was going to put on it with my 302. A t5 should do very well behind a 216, 235, or even a mild 261.
