I'm not a welder, by trade but, I am very good at weld prep.
In my young days, one of Gurneys welders, opened his own shop, in Orange, CA and taught me some basics on preping my alum & mag parts, before bringing them to him.
For 2 years, I did weldment preping and fitting, for high vacuum chambers, for NorCal Products, the largest high vacuum equipment manufacture, in the world, at that time.
Welds DO shrink the material being welded, anytime you penetrate, the parent material, with a arc. That's why you can remove valve seats by running a bead around them.
My job was to position tubular ports at different, skewed angles, cope the fittings and then tack them in place with a tig (no rod used), so there were no gaps (Hi Vac welding is done w/o filler rod). They had to be positioned +/- .010, after welding. I positioned them .010/.015 high. If I knew who was going to weld, I could set them closer.
The welders were required to maintain a welding technique (amount of heat input) to keep shrinkage to .005/.015, so the finished product was within the +/- .010 tolerance.
So, every time you lay a bead, that penetrates, that area will shrink, in that range. Laying short beads on both sides, will shrink less and evenly, laying a heavy bead, on one side will bend it, no mater the jigging. If that jig doesn't come out, like it went in, it's bent.
It also helps to stress relieve or, normalize after welding. Stress relieve at 600-900 F (use colored markers), normalize by bring to a dull red. 600-900 F can also be determined by metal color, if the metal is clean. 600 is blue, 900 black.