All TD and TF chassis are the same size with minor modifications over the years. There is no drastic difference in the TDs tub sizes, only minor, minor meaning fractions of an inch, but one will fit on the chassis of another. As far as I know, the location of the fuel pump from the firewall to the rear of the chassis on the TFs is the only major chassis change made from the first to the last. The Mark II TD has some improvements over the earlier models, but the tubs are the same.
You have to take into consideration that these car bodys framework are made out of wood with the sheet metal basically nailed to it. That's were the variation come into play. A lot of shimming and positioning is possible to get everything to work properly. It takes a lot of patience to restore one of these cars and it's not for everyone. As simple as these cars look to the novice, their a royal PITA to restore and be very costly if you mechanical skills and tools are limited. PJ