New to the forum and to A-bodies - advice on a build direction
It sounds like the tunnel needs to be opened up to support the 8HP70 transmission then?
I only know of one situation where the tunnel was claimed to not have to be cut, and he moved the motor forward. So I would say yes, the tunnel will need to be cut.
Something to know about the torsion bar suspension is that there is a crossmember over the top of the transmission that supports the weight of the car much like the shock towers in a Mustang. And it is fairly tight and almost always needs to be cut to fit a late model OD trans. If you keep the torsion bars and remove the center of that crossmember it really needs to be built back in somehow. There are those that feel that the bolt in transmission support is enough, but I like the idea of the car being able to support itself without having that support bolted in. This is also why many go to a coil over conversion (COC), it removes the stresses that crossmember sees and frees up room that the torsion bars take up, but it adds stresses to the chassis that it wasn't designed for usually by putting the weight of the car into the inner fender that was only supposed to resist the shock forces. Most (all?) COC kits take this into consideration, but some argue it doesn't fix it all. But that's the builder/owner to decide.
I think a 4L60 needs a cope at the front of the torsion bar crossmember but the tunnel itself doesn't need to be cut. Same with a 200R4, maybe? The A500/42RH/42RE can be bolted in without modifying the tunnel, but the crossmember has to be removed over the trans and a thick (1/2"?) plate welded to the floor and ends of the crossmember. For an automatic OD, I can't think of any other options beyond the 5 speed the earlier LX/LC's came with and it is much more complex to swap, needs the same floor mods and isn't as good a trans as the 8HP70/90.
One note; it is my opinion that the single biggest mod anyone can do to a car to make it more modern and enjoyable to drive is an overdrive transmission of some type.
