According to the Instructions that I got with my TTI headers, the CRANK CENTERLINE should be @ 2 1/2" offset to the passenger side, and 5 1/8" above the TOP of the crossmember.
I beleive all the A-Bodies 67 and up measure 30" inside of frame rail to inside of frame rail.
So tha means the crank centerline should measure 16 1/4" from the LEFT(Drivers) side, and 13 3/4" from the RIGHT(passenger) side, and 5 1/8" up from the crossmember.
I am in the process of modding a 67 smallblock crossmember so that I can use the biscuit mounts(more room than spool mounts). I am using these measurements and everything mocks up fine.
As stated in an earlier post, you MAY have to shim the motor mounts up a bit, I think Schumacher has a set of various shims available for this.
Yea, I know I can buy Schumacher mounts, but I like to do things my self. I am also retired and have more time than money. LOL
Herb