I wouldnt bother with a jig, but you may want to do it in sections- weld 3-6" then do another side then come back to finish after cooling some.
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'69 Trans Am clone under construction -still!
LS6 6speed
69 R/S Z28 Daytona Yellow 302
90 Formula / Firehawk clone
1999 Firehawk
1969 Chevelle SS 396
Check out the ATS website as they used to do this. I have tried to start welding on mine, but I need to get it cleaner before I can do anything with it. I would recommend only doing short stitches at a time.
I rewelded my subframe with a mig and it worked out fine. Its messy with flux core so if you have something that you want to visually see then I would suggest a mig with shielding gas conversion.
Shielding gas will cause cooler welds and possibly less penetration depending on what welder you use so I went for the flux core but I needed to weld in some inconspicuous areas. Im using a 220 volt mig welder now with a gas tip but I didnt use gas on the frame but I will for some tubbing and some other sheet metal work.
After some quick research I decided that fully welding the subframe was more for aesthetic reasons. So, we (my dad and I), just stitch welded the rails and concentrated more on the UCA mount. This was done using a millermatic 175 (220V) and gas shielded wire. Here's some pics so far. Its not done yet.
..... And we did not make a jig, just took our time.
My situation was different because I had to repair my subframe and I was going for full penetration. For stitching its obvious the gas shielding is better. I did a bit of stiching but nowhere near what you did but I didnt have shielding gas so I wasnt able to get as technical with it. Im still in need of a tank. Ive seen some very nice TIG welding and they get very good penetration.