Good on ya for following up with the vendor. The only way they have to resolve such problems is to get feedback from the people who are using their products, as a large majority of the guys selling the parts aren't old enough to have actually worked on these old rigs when they were daily drivers. Another sneaky trick is to use a piece of 3/32" oxy-acetylene welding rod as a probe to reach through the water jacket, then slip the end of the copper tube over the welding rod to guide it from side to side.
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!