You need to wait till you have the bell housing you're going to use. The flywheel size isn't tied to the engine. If you go with a T-5 then you're limited to a 157 tooth. The TKO and Toploader can use either.
As stated, the stock bell housings will dictate the flywheel you can use. If you opt for an aftermarket steel bell housing, it could be drilled for both sizes (157 and 164). This is how my Lakewood is (302/toploader).
You could just get an adapter plate to use the older style bell with a T5 or TKO cant you? With a stock 164-tooth (Galaxie/Failane) bell or aftermarket (you can usually use either flywheel) bell and the adapter you should be ok.
You could just get an adapter plate to use the older style bell with a T5 or TKO cant you? With a stock 164-tooth (Galaxie/Failane) bell or aftermarket (you can usually use either flywheel) bell and the adapter you should be ok.