Interchangeable parts between 1976 & 1978 engines?

[Mista750]

I am a new owner of a 1976 CB750 Super Sport. This will be my first CB750 and thought this would be the place to ask. The motor on the bike runs and according to the engine serial number, the motor was from a 1978 bike. I lucked out that the previous owner included the original motor with the sale. The carbs are the original 1976 carbs as well. The original motor has a cracked clutch cover, the top end has been pulled part and pistons removed. All the parts were bagged all in one box and looks complete. Unfortunately, nothing was separated and everything is mixed so I can't identify what part belongs with cylinder 1, cylinder 2, etc.

My plan is to put the original motor back and use the 1978 motor as a guide to help me rebuild the top end on the 76. I have a replacement clutch cover and once check the lower half of the 76 motor is good (transmission, crank, seals, etc.), I'll start piecing the top end back using the box-o-parts it came with. If there are parts missing on the 76, wondering if I can use parts from the 78? Can anyone confirm what parts are interchangeable, if any? Aside of not letting the timing chain fall back down into the motor during the build, any other advise on building the top end of this motor? Any help would be awesome! Thanks!

[TOOLS1]

The main difference between the two engines would be the location of the counter shaft sprocket. In 77 Honda moved the sprocket out 10mm to make clearance for the 630 chain. Pretty much everything else is the same. Since your bike has the 78 engine installed who ever changed it either changed the cush drive, and maybe the swingarm. Or they used an offset sprocket. When you go back to the 76 engine you will need to figure out what they did. You will either need to change the cush drive back, replace the offset sprocket, or install an offset sprocket.
TOOLS  

[Mista750]

Thanks Tools. The swing arm may have been changed. Makes sense since he also threw in an extra rear wheel complete with drum brakes. That wheel and swingarm may have belonged together. Also in the parts bin, I found extra parts for a swingarm. I'll ask if the previous owner if he still has the original swingarm.

Regarding the engine, since everything is the same, can't I just take the topend (piston + engine cylinder and up) from the 78 and simply transplant them onto the 76? Or would that be a bad idea?

[TOOLS1]

It would work, but why would you disassemble a running engine to rebuild another engine? If you take it apart you are going to need to at least re-ring it, and then the rings will seal so well that you will have blowby through the valves, so then you will need to have the valves done. Why not just rebuild the 76 engine? The SOHC 750 engine is too easy to work on. All you need is the factory service manual (which can be downloaded for free from the manual section here) and you will be able to reassemble it.
TOOLS

[Mista750]

It's probably the OCD in me that wants to have the original motor back on this bike. But you have a good point sir! I'll just wait till the 78 motor craps out (which will be probably be a while knowing the history of this motor) and then rebuild using the 76. At least I have a motor I have tinker on while I ride. Thanks for the help!