I haven't ever had a problem with a 299, 318 or 320 myself.
The stock 12 is great for what I am using it for, but I wouldn't want to go on a long trail ride with it, tops out at maybe 35 on the hard pack.
71 was the first year but they made them a bit too light, this one had been broken and buckled at the bulkhead in the past (note the big, riveted patches and inside there was some brazing) - 72 up they added bracing to the front of the tunnel and stampings in the sides of the belly pan to stiffen it up. Uses the same track as any 15" wide by 114" bogie wheel Oly or TNT.
Great machine to go exploring with, I can load in the van myself with no ramp easy.
I still want to get some wider bars than the 71's and made the post about 4" longer for riding standing without having to bend over.