Welcome to the forum Alan.
If you want to be able to install a Zetec, the top of the round front crossmember (the piece that acts as the vacuum tank) has to have a half round cutout to accommodate the Zetec harmonic balancer / front pulley. This is in addition to the notch shaped cutout in the back side of the crossmember that accommodates the length of either the Lotus TC or Zetec.
This was not 100% clear to me when I bought my frame several years ago, and I ended up with the Twin Cam only frame. I recently judged it too difficult to modify the frame later to add the notch; probably body removal, steering rack removal, brake line removal before one starts cutting and welding. I have installed an MT75 five speed but I am sticking with a built up Twin Cam due to the chassis.
So, although the guys saying there are the three basic frame types (folded metal like the original, Mk 1 Twin Cam Only, and Mk 2 Zetec) is basically correct, it is a bit confusing. They don?t necessarily make every new frame automatically to accommodate the Zetec, at least in my case they didn?t. And every Mk 2 vintage chassis will not be the same. Basically mine has all the mods except the half round cut out.
For fitment of the MT75 five speed (the transmission Spyder uses for their Zetec conversions) there are two required modifications. There are (from memory) six captive nut / bobbins welded in to the bottom frame rails to accommodate two different transmission mount positions with four bolts each. The other mod is either bending or relieving one of the ladder frame cross bars to accommodate the starter motor for the MT75. This was not done on my frame, and I modified it myself by cutting the crossmember out and replacing it with a bolted in channel.
This picture shows the half round cutout and sort of shows how the right side diagonal member is bent outward for the starter. It doesn?t show the notch cutout very well, it is easy to see looking straight down at the crossmember. Looking over the other features, this frame is set up with the double wishbone rear suspension. I can?t recall if my frame came cut out for the tunnel handbrake, I doubt it.
To take a look at other pictures, try clicking on the photo gallery, change to showing 800 pictures per page, and slowly type in a filtering term like Spyder or Zetec and scroll through the pictures. You have to type slowly as it filters with every key stroke. Not a fast process, but you can see what is out there, as there are certainly other approaches and solutions used.
But for a quick visual check of a candidate Spyder equipped Twin Cam Plus 2 runner, I would look for the half round cutout as it is easy to spot under the bonnet and very difficult to add later if you want to. Or just say to heck with it and look for a nicely built Zetec.
That would be my preference, but they are not really available our side of the pond.
HTH
Stu