Which 4-speed?
You'll probably need, all the parts on the transmission output shaft of a 4-speed, an input shaft gear from a 4-speed up to March '75, different propeller shaft lengths (flange patterns must be correct), the shift shaft and matching parts from a 4-speed, transmission hump (maybe), bolts from the 4-speed transfer case to transmission, and a 4-speed bellhousing (flywheel and clutch with slave pushrod can remain, but they must match). On early USA 40s the frame crossmember behind the parking brake drum is tight, and often requires the engine to be unbolted at the front mounts with the water pump fan cleared from the radiator shroud so that the engine can be tilted forward (you should measure your clearance because it is a '77). Expect gear wear on the 4-speed output shaft, and gear, which could require the use of an oil bypass hose from the transmission to the transfer case so that it doesn't overflow.
The output shaft of a 3-speed in my shop is perfect. I have two 4-speed transmissions that show normal wear at these locations and will probably leak oil from one box to another, making the bypass hose kinda necessary. The 3-speed has no synchronizer on first gear, so it makes downshifting hard from 2nd gear. The ratios of the 4-speed H42 are not that much better, the H41 is a whole different story. I know that this doesn't answer your question, but it is some things that I have considered.
Welcome to Mud.
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