Turns out we had all sorts of trouble fitting the new engine in. The 4L60e if quite a bit fatter than the T700 so out it all came and took to the tunnel with a grinder...
After much discussion we decided instead of fabricating in a VR Commodore tunnel Holden202T had spare, we should try and modify the original tunnel and see what happens. The gearbox mounts were the main concern if we cut out the old, so we bolted up the cross member back in place for support and to maintain its position while the modifications to the original occurred.
Original tunnel
Cutting
As a result, we sliced down the middle of the tunnel and in a few places across the front to make it wider and taller. There is also more room now for the intake manifold and coolant pipe...
Welding back up
Then I ran out of gas

so I'm off now into town to get a bigger cylinder.