McGyver----OK--get the 1 3/4 die, some 1/16 thick sheet stuff, use it for a shim with the 1 5/8 tube, it'll form into the die---

BUT, as others have said--get the tech manuels and specs, because there areany different types of TUBING ( and even PIPE ) and most likely what you have isn't legal--if you got it cheap its probably welded tubing and what you'll need is seamless-----

also the design, where certain tubes,braces,gussets are fastened make a differance not to even get into the welding part of it.