Your overdrive options are the same weather you go with the chebby or the gmc motors as the bellhousings are interchangeable. It may be a little bit more tricky to find a flexplate for the 270, however.
I would pic the T5 over the automatic, but it isn't your only choice for an overdrive transmission. The A833 4 speed, and the NV4500 5 speed are adaptable to the chevy bellhousings.
The NV4500 gives the advantage of a granny gear to get the load rolling, and a .73 OD. It is a big and stout transmission that probably would not be needed in your application.
Another MUCH CHEAPER OPTION would be to use a 3 speed + overdrive transmission like the ones they used in the 50s.
The T-5 is probably the best performance for the buck. You will have to have an adapter plate made if you decide to use the '41 bellhousing. You may want to use that bellhousing because that is where the brake and clutch pedals mount. You could use a 47- 54 bellhousing that has been modified to accept the brake and clutch pedals, but then you would have to lengthen the splines and cut the snout of the T-5, so the use of a 1/2 inch thick adapter plate is actually a bonus.
