Nice ride. Going to be sweet no matter how you outfit it.
Looking for a matching motor and then using a 72 6 pac makes no sense to me.
I don't care about numbers. If it aint original, it aint original. Just my 2 cents. As for the 70 cast motor. Seems to late unless the cars build date is much later in the year. Didn't they start production of the cars themselves in late 69 anyway. I think the motors were cast and built before hand to be ready for assembly. I would go with one of the later 69 cast blocks.
I bet you got some weird looks doing the wash. I would never have thought the frame rails would withstand the weight but again I would have been wrong again.