I finally got round to fitting the gearbox yesterday. Getting the old one out was probably more difficult than putting the new one in. The Avenger box has little wings on the bellhousing which makes it very tight in the tunnel on removal (in retrospect I should've removed the prop cradle before the gearbox).
The tailhousing of the Avenger box sits as below, with the flat piece of wood placed across the floor just in front of the prop cradle.
The main reason I actually wanted to change the box was because the 4 speed one leaked a lot of oil, and every time I parked the car it would leave a little pool beneath it. You can see some buildup in the bellhousing after I took it off.
I had to take the clutch off so that I could fit the new pilot bearing for the Ford box.
I had a spare Avenger input shaft which I wanted to use to align the clutch with the pilot bearing. I had to turn down this extra bit of copper pipe and tape it on so that it was a good fit in the Ford pilot bearing.
After removing the prop cradle, the gearbox went in pretty easily, although it can't sit with the tailhousing at the same height as the Avenger box without modifying the tunnel. At this stage I'm uncertain whether to modify the tunnel or leave it a little lower.