I mark the spot welds with a punch and use a Blair Roto-Broach
https://www.blairequipment.com/rotabroach-cutters to cut the spot welds. You can get several different cutters for the same arbor. Not cheap, but if you use them carefully (keep it square and don't wobble) they last longer than other cutters I've used.
Practice making plug welds on something that isn't your 914 (using similar gauge metal), before you want to weld the tray in. When you are good and ready, practice some more.
Fit the pieces carefully and make sure it's properly located and the mating surfaces line up good before you tack anything in place. I drill and dress up any holes for plug welding, so you get a good fit.
I use weld through primer on both surfaces, my favorite is U-Pol #2. It's not for the actual welds, but IMHO it's the best thing you can do for the metal surrounding your welds. I used a dental tool and scratch off the primer in the holes before I plug weld them. You don't have to do this, but I've found it makes it easier to strike an arc and weld.
The hardest part is welding the tray to the inner fender wall, and if that wall isn't solid it adds to the difficulty. You can make welds along the top edge along the wall, but it's hard to get in that gap.