sill replacement advice requested please

I recently received all of my sheet metal components to replace both complete sill assemblies. As I look at the new parts and how they fit together, I am wondering if I could weld 2 of the parts together while on the work bench before installing this assembly and then the remaining pieces into the MGB. It appears to me that by welding these two parts together first, the new assembly would be more rigid and therefore easier to properly position to insure proper alignment. For clarity, I am speaking about parts #12 & #13 from page 61 of the Moss Spring-Summer '07 MGB-75 catalog.

(Moss part #458-440 & #458-280 for left side and #458-445 & #458-285 for the right side) Thank you in advance for your input. You guys are great!

I've done it once. There is a certain amount of "fudge factor" during each step. FWIW
I wouldnt weld them together first as it totally removes any adjustment between steps. They wont be any stronger if welded first because they're going to welded together eventually

Right, what rrmg said. You can weld together on the car after fitted and clamped in place. See link to project and look on the sills page.