The 450 was only offered with a 302. The cab is identical to a pickup. There is a filler between the cab and the butterfly hoods that give it the extra length. The butterfly hoods are unique to the 450 thru 600. The 550 and 600 would have an Olds V8 engine. The 630 would have a longer hood to accommodate the big bore inline. The 2 speed is probably an electric Eaton or it could be a Timken. It probably has a twin disc clutch which will not mate up with the SM420. The transmission would either be a Clark 205V or Clark 205V or VO which would be OD or a New Process 540 direct or possibly overdrive. If it hasn't been altered, the specs will be on the glove box door. Hydrovac brake parts are still readily available. As for the wheels, it is hard to tell from the picture, but they could be stud piloted discs or Dayton with 20" tires. Both of these are very common. As for the gas tank, it is highly likely that it either never had a seat tank or the seat tank was never used. Since these trucks got less than 5 miles to the gallon, they usually had larger tanks.