When the original MG Midget crossmember is cut out (see Tunnnel Mods), you remove a pice of crossmember which is just over 7” (175 mm) long.
The inside size of the crossmember is 60 mm sq plus a smidgeon. This means that a piece of square thick walled tube, 60 mm square x 7” long, can be slid up each crossmember to act as a reinforcement and to which the new gearbox crossmember can be bolted.
Have you ever tried to get thick walled 60 mm sq tube? Well, unless you are really lucky and you happen across some, you can buy it in lengths as short as 5.3 meters!
I also don’t really like the idea of bolting up through the sides of a hollow section (even thick wall section) without some kind of reinforcing tube or suchlike inside. The sides are bound to give when tightening the bolt.
I didn’t have any 60 mm tube but I did have 20 mm tube.
3 x 20 mm =60mm. Hey presto.
I made up a honeycomb section with 9 x 7” lengths of 20 mm sq tube. It also much less crushable. 20 mm square tube (or 3/4”) is universally available. I welded the ends of the tubes to keep the whole lot together. Then, with a few taps they slid up inside the open ends of the remaining cross member sides. The pictures below show it all. Dead easy.
You do need to make sure that when you drill through to bolt the new gearbox crossmember on , that you don’t try drilling all the way down one of the “webs”. You can easily break a drill bit that way.