First task was to drill a couple of 10mm corresponding pedal bracket holes in the drivers side pan (the original holes appear to be 11mm, but I think the smaller hole looks neater). To get them in the right place I clamped the floor pan half into place and then sprayed a small amount of red primer through the tapped holes of the mount bracket. This gave me the markings I needed for my holes:
Next up I drilled out a series of holes spaced 25mm apart around the edges of the pans in preparation for plug welding. For the narrow areas (where the pan did closely follow the profile of the tunnel) I drilled 6mm holes and elsewhere I went with the standard 8mm:
Also added a few holes for plug welding the pan to the topside of the reinforcement plate, just as the original was attached:
Then I cleaned up around the tunnel lip with the drill mounted wire brush and shot blast the small localised areas the drill could not reach. Once all traces of rust were eliminated I applied a liberal coat of zinc/weld-through primer.
On the passenger side pan I chiseled off the unneeded captive nut (which would have been used to mount the pedal adjuster on a later model LHD car) and then welded up the hole:
With the drivers side pan fitted up the best I could do, I began adding the plug welds. It was pretty slow going as I needed to scratch the primer out of each hole to be plugged with a scribing tool. Obviously the added benefit of taking time also ensures that heat doesn't build up to a point where things start to warp.
To ensure a tight fit I enlisted the help of a couple of friends (thanks Dangerman & Chimps) whenever I could grab either of them for an hour. They had the important task of standing on the inner lip edge of the pan ahead of where I was about to weld and would lean their weight on an inverted broom stick. This proved to be an effective way if getting a flush fit.
Drivers side pan in place:
Fortunately both of the pans pulled in tighter to the tunnel as I progressed, so the gap I was previously concerned about was less of an issue. In fact, it pulled so tight in places that I needed to rely on some hammer persuasion to keep things flat!
Passenger side pan in:
Finally got it all mounted back on the rotisserie ready for clean-up and a few final touches: