I don't know how well it would adhere on PC, but
JB Weld is a pretty tough epoxy I've seen in use to make custom tube chassis' for RC cars. I've also did 'cut n' shut' driveshafts with these (cut them in half, take a bit out, put epoxy on and put the ends in a tube for surface area for bonding and of course to make them straight). I used these on a
3000kV powered 1/12 Monster Truck and they held up all the abuse (not just in snow, also on gravel and tarmac), to give you an idea of the strength.
I have never lengthened or shortened a chassis like this, but what I think would be most important doing with a good cut n' shut chassis is to make sure not just the ends (wall thickness of the chassis) are bonded, but there is some material overlapping both parts of the chassis (plates?), to which it bonds. It makes for a much stronger, more rigid chassis.