When converting from SBC to BBC with a 4spd, what is the determining factor for changing the crossmember. Seems like a lot of previous post say you "might" need to change the crossmember.

If you are refering to 67-69 the factory did use a different crossmember for a bb. The trans mount holes are offset to match the offset motor mount brackets.

Some people re-drill the sb piece or use a later trans mount that has one mounting hole.

Sorry, yes it's a '69 I'm asking about. So, since you can drill the SB version, would the only reason to need the BB version be for a correct restoration.

The SB one will work if is redrilled. I would get everything together including the headers. Then move the trans to the right until you get the best header clearence. In a BB car the header to steering box clearence is the biggest problem.