Read up as there may be some other parts like a sprocket carrier or such that you would need to line up your chain with a 73 frame and 78F engine. Never done it here but plenty of threads. Also pistons etc would not be the best situation for you for parts swap into your K3 motor.
the late model motor you are looking at has a few issues before you can use it in your '73 frame...
1. The countershaft sprocket it located 10mm outboard from where yours is on the '73. You can get a 10mm offset rear sprocket from CycleX or you need to obtain the sprocket carrier and wheel spacers from a 77/78 cb750. The sprocket carrier from the f bikes (like the one pictured in the craigslist ad)will not work because of the rear disc and it wont fit your '73 hub anyway.
2. This will move your chainline 10mm outboard and clearance for your rear shock will be extremely close. You will need to convert chain and sprockets from the '77/'78's 630 sized chain to the '73's 530 sized chain. This will work with oem shocks, anything thicker than the oem shock and the chain is gonna rub on the shock.
I know all this because I am running a 78k motor in my k5 frame right now.
