I had an Enticer longtrack of the same vintage as the one you have in the pic. Bulkhead was shot (rusted out from sitting outside with leaves/pine needles in the belly). Also had a 78/79 (red/white) ET340 who's tunnel had been wreaked.
Built 1 sled out of the two. Everything was pretty much bolt/rivet and go using the necessary parts from the longtrack and just parking the all behind the other. Worst part was the factory rivets Yamaha used to marry the bulkhead to the tunnel on sleds of that era. They are a pain to get out without wreaking things but seeing as I was only saving the bulkhead from one and the tunnel on the other I just ground the rivets from opposite sides to save the parts I needed.
Worked great and was not that hard. Sled lasted for many years until the throttle stuck open one day when the guy who had it started it up. It took off, went across an 80 acre field and disappeared over a hill. He followed the tracks and itt was found smashed into an oak tree idling. I wasn't there but I guess it went through a few brush piles getting diverted in multiple directions before the big oak put the stop on it. Front bumper was caved back to the crossmember, hood was junk, 1" stick was driven through the cooling shroud and wedged half way through between the cylinders, seat was torn off and track derailed.
He drove it home but that was it's last ride.