Buy a flex hose to fit the larger diameter. Be sure it has a reinforcing wire to prevent collapse because it will be on the suction side of the water pump. Cut a 2" long sleeve of the smaller diameter hose and install it DRY on the radiator nipple. Slip the larger hose end over the sleeve, also dry with no attempt at sealing it, and use a worm screw type clamp, tightened firmly. I have used this method on racing cooling systems with 20 LB. pressure caps without any issues. No need to pay ridiculous prices for a custom hose when low tech will get the job done, unless you're some sort of anal-retentive purist. We don't have any of those folks on this forum, do we?
ohwell
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!