Paul, not sure if you are trying to reduce an 1 5/8 on to an 1 1/2 or stretch an 1 1/2 on to an 1 5/8 however your right that it's not a good idea. You want the integrity of anything that can leak dangerous gasses or water into the boat to be done correctly so it is safe for you and the crew. IMHO.
You could maybe cobble an adaptor together or find one.
Dave Neptune