Well Mickey I had exactly the same dilema some time ago on my 740 with the same or similer plastic rad. No clamp would hold because there was not enough neck left. I did jury rig it by using some, wire and duct tape to pull the hose towards the rad, lots of silicone and then partially tightened the clamp. I left the reservoir cap loose so no pressure build up and carried lts of spare water. It got me home and lasted for a few weeks after that. I won't say I could recommend this but it worked out for me, but I am a bit of a risk taker sometimes. Whatever you do, don't let it overheat as you may cause serious damage such as blown head gasket, cracked head or worse. Later I replaced my rad with a 3 row Nissen all metal.

Good Luck