I was the Prod Mgr at a factory that made a lot of oem stuff for harley . they had a computer controlled bender thad had inner mandrals, balls connected with a Ss cable it pulled thru the tubing as it bent it . about 150 k for it .I was always told to pack the pipe with dry sand weld caps on the end and heat and bend it . I tried it once and it didn't work. I built a set of outside exhaust for the 427 Cobra that set the track record at MIS. The car owner knew the owner of the Thrush mufflers factory in detroit and he let us use his shop from 6 PM to 6 in the morning Every kind of tool to fabricate you could dream of. the easiest way without propper benders is to buy prebent turns and cut and weld