I'm not sure how much longer the 79 swingarm is, I'll measure it tonight. Off the top of my head I wanna say it is 5"-6" longer but I am not sure.
Adding to the swingarm is not that unsafe as drag racers do it all the time. There are plenty of modern street bikes roaming around with lengthened swingers also. The problem is finding a shop that does the mod. Personally If i was going to go crazy enough to lengthen a stock cb750 swingarm I would lop it off at the pivot and build a new square tube arm around the rear wheel I wanted to use (imagine using a fatboy rear rim with stock harley spacers on the cb750).
From a builders stabndpoint it is actually easier to build a swinger from scratch than it is to add length to an existing one, espically if the existing one is 30+ years old and round tube. All you need is a flat surface, a level, a straightedge, a ruler, a welder, a chopsaw, and some square tube. If you know what your rear wheel and axle is going to be you can even use the adjusters off that bike (I am partial to pre 1990 fzr600 rear adjusters when dealing with square tube swingarms).
