Based on the diagrams in my 1969 shop manual, there are some slight differences between the two models. The wheelbases appear to be the same. I think 68 and 69 are the same generation platform, but there may be differences. Find a copy of the 68 shop manual(s) and examine the diagrams closely. It will almost certainly require some modifications to make it work. Compare the frames side-by-side if going to attempt this. You probably should avoid a Mercury frame. It looks like they have a longer wheelbase.