the cross member will fit
the spool mount will fit
but some cars have an indented section to the chassie member on 1 side for 1 of the bolts.
and this was replicated in 1 side of the orginal cross member
this is to help you avoid putting in the cross member and square early mount on backwards
if yours is like this would be wise to pack the indented section with washers so that when you do the bolt up the later one you are not crushing the cross member.
dunno what year is cut over year for this in the US
but 71 cars were still like this in Australia due to use of the mid 60s auto trans floor pan tunnel and front chassis section until.....? well....the end i think.
i am assuming 68 had a 1 or 2 bolt square rear trans mount on a year specific square rear mount, cross member, and you want to go with a 73 onwards cross member and the round rubber mounting with the bolt through it.
I have the parts to do this but discovered that painting my rear rubber mount with black gloss spray paint (the kind you use on iron railings) seemed to help with the "it got oily and went soft" problem. i.e its been in 10 years and has not gone soft so the spool mount upgrade hasn't happend
Dave