The big difference is the fork. 35 to 38 and up to 11.5inch. The 78 enduro is mostly the 77 MC. There is not much difference in the rear suspension many parts fit from one to the other. The 250 got an extra boost port but apart from that the engine is the same and you can port a 77 barrel to match anyways. The side covers are a little different and wrap around the shocks more but you can fit either side covers to either model. But I dont think you can argue for follow on model. A 78 enduro with 77 forks would go unnoticed mid pack im sure. I did buy some parts from a guy on the penton forum who had some 77 frames thou. And you can fit 77 YZ forks. He did have one with forks fitted as well. Great motors those 400's