I've been reading Morris' posts and checking out his build progress for a while. He's got some inovative ideas and beautiful fabrication work in that car.
My Classic Revival chassis is very different to the Kirkham. It's a 4" square tube design and the tubes are spaced wider apart. The T56 was a fairly straightforward install. The tail of the gearbox hangs over the top of the main rear cross member. The cross member is a big X of 3" square tube welded between the rails and the rear gearbox mount is in the right in the V of the cross. The only real problem is to get the box out you need to pull the motor. I'm tempted at some point to make the tunnel removable to make working on it easier.
You can sort of see how tight it is in this pic:
Cheers