In general the tri moly frames has so so welding on them, whereas the full cr-mo frames had much better welding.
Your work is awesome!!
The problem with the Moto Mag frame and many early BMX specific frames were that they were designed to be used with 46/18 tooth gearing and shorter 167/170mm crank arms, and much larger diameter rear gear that would keep the chain away from the chain stay. but in time when the flip flop hub became popular and our crank arms became longer (170mm to 175/180mm) our gearing went from 46/18 to 44/16 because of the longer crank arm with the theory at the time was to change the gearing to a more respectable size to counter act the longer stroke of the crank to enable the rider get the bike rolling. So when we would run race gearing with flip flop hubs 44/14 a smaller diameter rear gear made the chain come unbelievably close to the chain stay, and usually ended up rubbing through the chain stay over time, just from natural chain slap. Have you ever thought of doing side work repairing old BMX frames? We used to have the chain hole welded and re ground, a cheap fix but somewhat nasty looking. Changing the whole tube, I think you would do a awesome job. Just an idea to save older frames from the scrap yard. Very nice work and build!!