There's a simple, quick fix that I learned while working in a radiator shop. Buy a flex hose that fits the bigger diameter, such as the 1 1/2" thermostat housing, and cut a short section of 1 1/4" hose to slip over the upper nipple on the radiator. Install this piece on the radiator nipple WITHOUT SEALER OR ADHESIVE and slip the larger hose over it. Clamp it snugly with a worm-drive hose clamp, and it will be secure and leakproof. Do the same thing with the lower hose, adapting the larger diameter hose to the smaller radiator nipple with a reducer sleeve. This has worked for me on race engines with 20+ PSI radiator caps.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!