Actually the method of shortening the VW pan in the build manual is not that bad. I know it sounds hokey using wood 2x4" for the "precision" alignment tool but it works. Once aligned tack weld and measure again. I used a oxy/gas torch to weld the tunnel. That worked and I was able to use the heat to get metal to bend and fit tight. I used a mig welder for the floor pans. A cheap HF angle grinder is indispensable!
I recall seeing that body on ebay for quite some time - aren't the kick panels cut out/missing. Plus looked like the car has the old butt sag too. Did it not come with a title for the pan? Most all derelict VW need new floor pans and they aren't difficult to install. Since a pan has to be cut completely in 1/2, you can mix a super beetle rear with a std beetle front piece. Just remember the rear part is what has what passes for the VIN.