Originally Posted by MARTINSR
What transmission does your Rolls have in it originally? I believe they had a Buick Nailhead ST400 which is a TH400.
The Rolls auto in the Silver Shadow is essentially a GM TH400 but has a different front bolt pattern. Internals are virtually the same bar some machining of the case to improve shift quality. Back in its heyday GM Powertrain manufactured these trans with a universal circular bolt pattern and supplied these to Bentley, Rolls Royce, Jaguar, Ferrari and others who made their own adaptor housing between their engine and the GM trans.
I have seen a crowd in the US that makes an adapter between the rolls trans and the GM engine but I'm not a fan of running spacers etc between flex plates torque converters etc as has seen way to many of these fail. Benefit of this method is that it still uses the Rolls adaptor housing where the trans is mounted as they don't have a crossmember at the rear.
Other options are to run a heavy duty GM bellhousing case as the drag racers do that requires you to cut the front part of the RR case at the pump to bolt the new Belhousing to or swap the case entirely with a GM case. Doing so will however require rear engine mounting brackets for the SBC that line up pretty close to the original rolls mounts.
Sounds like a lot of mucking around but the benefit is that you can retain the RR electric trans shift on the column which is such a great lazy mans feature once you have used it.