The Rover SD1 gearbox is renowned for having a poor gearchange particularly when cold. The guy who sold me this gearbox had invested nearly £2k on a Borg Warner T5 box as he was fed up with the gearchange on this R380 gearbox. (The R380 gearbox is based on the LT77 SD1 box and was fitted to the later MG RV8). I don't know if my method for gearbox mounting would work on a LT77 gearbox.

I removed the mounting plate from the R380 gearbox and found that the hole spacing for the mountings was about 4mm narrower than for the MGB mounts. The mount angle and position was correct for the standard MGB crossmember, and the gearbox sat at the correct height with the MGB crossmember attached. All that was required was to widen the mounting holes in the mounts to suit the spacing of the mounting holes on the gearbox.

The completed gearbox mounting is pictured. The old position of the crossmember is just visible on the chassis rail to the LHS of the picture.

I made some changes to make it easier to fit and remove the crossmember: The mounting slots have been lengthened so the gearbox mounts can be mounted to the gearbox and easily slotted into the crossmember. I've also made some large holes in the underneath of the crossmember through which I can insert a socket to tighten the mounting nuts.

The gearbox crossmember had to be mounted about 4 inches further back on the chassis. I cut a hole in the floor to access and remove the nut plate. I drilled new holes in the crossmember for the gearbox mountings, then re-fitted the nup plate in a new cage. The plate I welded into the floor to cover the access hole can be seen to the left of the photograph. Incidentally, the new metal to the right of the photograph are rails I installed to increase the seat mounting stiffness as the original mountings had bent the thin metal of the floor panel

The SD1 gearbox is taller than the standard MGB gearbox so the transmission tunnel had to be raised. I wanted to avoid angle grinding and welding inside the cabin as much of the interior and glazing was fitted. I cut the small vertical section between the two horizontal tunnel panels out with a cold chisel, then planished the lower section of the tunnel upwards to meet the upper section.

The photograph also shows the pipework routed inside the tunnel to clear the LH exhaust pipe.