swapping to a big block from a 6 cylinder 4 speed

[kxnomore]

Forgive me if this has been posted a million times...

The truck is a 1969 C-10 shortbed. It currently has a 350 installed in what seems to be the original 6 cylinder mounts. I believe this because the engine is very low and far back in the engine compartment. But the clutch and transmission works great, and looks to be in the original location, based on how the bellhousing is bolted to the crossmember... My plan is to install a 402 I have from a parts truck, using new big block mounts and towers, and to keep the 4 speed.

What challenges are there in keeping the riveted in crossmember from the original straight 6 and 4 speed transmission? do you have to cut that crossmember out and would the transmission have to be moved backwards or forwards?

The main concern I have is the transmission crossmember that is riveted in, seems to be necessary to support the bellhousing etc, and seems like it would be tough to install the big block without taking it off. Do the big block towers actually locate the engine in exactly the right location? What have you guys done when keeping the original granny low 4 speed for this conversion?

Any suggestions would be very helpful, thanks!