I'm in exactly the same situation. You've started me thinking. With the original, you know the parking brake cables will be dead on. To lay out the T5 mounting holes, the T5 would have to be in the car with the shifter installed. That is so you could tell if the back of the T5 was sagging down and needed to be elevated any. Once the T5 was positioned, trial fit the original crossmember. If the bolt holes are so far off that they are into the flanges of the crossmember, you'd have to cope out the flange, weld in an extension, then weld the flange back on with carefully formed angles so that it would be strong enough and not look hoky. Then locate the bolt holes. Or you could peel off $179 and buy one.

 

I'm thinking I might be able to get away with measuring from
the back of the installed bellhousing to the installed stock
crossmember _before_ the T5 is installed. Then transfer that
measurement to the T5 and see how they compare.

And I don't quite get your comment about the parking brake
cables. On my '68, the parking brake cable doesn't seem to
connect in any way to any part of the transmission. Is the T5
going to change that?

 

The T-5 requires a mount that is lower than the original, thus you cannot just relocate holes on the original. If you have a few hours and access to a welder, you can make one for about $10 in materials.