Ditto on the above advice- - - -it's almost exactly the way we built dirt track cars when we would shorten a frame to achieve the minimum wheelbase for a car, and/or install a new front frame assembly after a major crash. Another trick is to get a set of screw-adjustable leveling jacks used for camper trailers and support the frame pieces in several places while mocking up the assembly, then tack-weld everything and re-measure before making the final welds. It's a lot easier to grind away a tack weld and reposition something than it is to cut out a fully-welded mistake!
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!