My certification is LTRF ( Long Time Redneck Fabricator)
First rule of redneck fabrication is never to ask for advice on how to do it. If you have to ask you are chicken yellow.
Second rule is that all testing is prefaced by the phrase " Hey Hold my beer a minute"
Third rule is all testing must be videotaped, If the testing fails and you hurt yourself in an amusing way the video goes on Youtube. If the test is a success no one really gives a sheot so don't bother posting it.
About your project. I did this project in 1990. My cousin Roger is proportioned like the missing link, his pants are like a 28 inseam and his shirt sleeves are 35" He wanted a MTB frame that didn't need a 200 mm stem to fit him. So for 2 doobies and a 6 of Dos Equis I took an old 22" Muddy Fox (Tange
#2 )frame and cut it top down to a 17" frame.
First I cut 5" from the seat tube, then I cut the head tube down 3" (I couldn't go lower without buying a new fork). I then separated the seat stays from the rear drop-outs. Next I carefully cut the top tube away from the stubs of seat tube and head tube. By sloping the top tube it makes the tube shorter so I had enough length in the old top tube tube to hand file the miters. Next I separated the seat stay( mono stay) from the seat tube stub and filed it down to fit. With all the fitting and filing done, I used my trusty Benzomatic MAPP torch and some Harris Stay-Silv 15 rods to braze everything together. Roger still has that bike. I have a picture of it, if I can find it tomorrow I'll post it.