Crossmember Question?

[Craftsmen]

1970 SWB c10 250 I6 3 speed manual:
Can some please tell me the difference between a 250 6 cylinder 3 speed bell housing cross member and the v8 3 speed bell housing crossmember?
I understand when I remove the i6 and install the small block in the 8 position I Need to purchase the v8 perches and use the forward set of holes moving the engine forward a couple of inches. I'll also have to get the longer drive shaft. I'm keeping my 3 speed tranny/bell housing etc. I understand that the cross members are different, it just is not a matter of unriveting the cross member and sliding it forward a set of holes and bolting it into the eight position. I really can't see why the part would be different, it's mountiing to the same bell housings just a few inches further up on the frame. Unless the frame is that much different in that spot? Can someone please help me understand this? Also if I need the 8 cyl. 3 speed crossmember where can I buy one? Thanks for any input.
P.S. I do understand I could just bolt the v8 into the six position and be done with it but I want the truck to look like it came from the factory with the eight cyl. Engine.