FWIW, I went to the local plumbing supply place and
found some copper tubing that fit snugly inside the
neck. Cut it to length and scalloped away a large section where the tube was inside the radiator (thus
using the remaining section for a depth guide). Glued this in place, then used two wide hose clamps on the
outside of the water hose. It's held up so far.
Later.
--ml