I would go with the 74. The 77,78 era are a good performing motor but these are the goofy years that honda was changing some things around with the cylinders and heads and can be confusing and harder to find the right parts. If you can get the 78 for really cheap go for it because it would still be worth it, but my choice would be the 74 motor. How many miles are on the bikes? Check see if oil is really dirty like it has been neglected. Look for major oil leaks because if you have to fix a head or base gasket then the motor has to come out of the frame. Timing chain tensioner cant really be inspected but is a maintenance item that is part of a good tune up. You will have to go over the tuneup when you get a bike, plugs, points/condencer, set timing/dwell, adjust timing chain, adjust valves, oil change, sync carbs, etc.