Well I don’t claim to be an expert on much of anything, but I have welded (and not welded) many frames.
With heat treated or low alloy high strength steel, It’s not recommended to weld on it at all, I have out of necessity and it’s broken close to the weld every time, even with a fish plate. Best bet is bolt on a plate. Keep in mind, most of what I’ve done was crack repair, flex any metal enough and it will stress crack, so if it cracked the rest of the material has been flexed plenty and it broke where the forces are compounded.
Reading the first post it looks like your boxing in the frame to make it stronger or more ridged, (correct me if I’m wrong) and in 53 I doubt the frame is anything other than mild steel, but it would be worth researching.
If it is Mild steel, I’d probably Mig weld it if it were me, less heat.
Just my 2¢.
Also a note on rods, thinking back the first 2 numbers after the E for electric denote the tensile strength of the rod compound in like thousands of pounds per square inch, mega Pascal’s, nuton pounds or something like that, but E70XX would be stronger than E60XX, the last 2 numbers are to do with the flux type or whether it’s AC or DC, it’s been a long time, perhaps someone with some fresh schoolin can help more with this.
Generally 60(11) works better on dirty metal and will penetrate better but is not as strong (or pretty) as E70(18). Sbeen my esperence.