I would think if you had a complete stock neck casting with the tubes cleaned out, and you carefuly cut the other neck casting off the frame tubes (leaving the original length frame tubes) it wouldn't be that hard to fit on. You must get rid of the two upright bars from the backbone to the support bar before taking the old casting off (wouldn't allow the gas tank 'bubble' to fit thru, anyway). Then, by starting the down tube fully into the casting first & pulling the downtube out a little, there should be enough space to fit the support bar & backbone into the casting. I would also drill 3/'8" or 7/16" holes thru the casting where tubes fit into, so you can weld the tubes to the casting more sercurely. Tack it in, and MEASURE TWICE for straightness & gas tank mount fit. Weld a couple of holes and MEASURE AGAIN. If all is good, weld a couple more holes, & LET COOL. Do some more 'till done, allowing the frame to cool before welding again. Less warpage that way. Goodluck. Doc