Re: power head swap?
Area's to think about and compare, outside of bolt pattern of bottom of power head.
control linkage mounting , shift linkage, (cable and shift rod), wire harness connection. and the trouble -some solenoid selnoid.
Also how far of a swap? are you using the same fly wheel of the power head, 76 the 76 flywheel 76 the 76 power head . because there could be a difference between the years .
lastly I would count the spline of the drive shafts and the end of the crankshaft, one year could be a fine spline where the other year may carry a course spline.