I used an 80-82 cross-member under my 77 for my LS transplant . I read that the 4L60 bolted straight on to it , well it did after I made a new tongue to suit
I used it because the 77 cross-member has the two holes but the position of the 4L60 pan would have meant some tight turns in the pipes which I could do with out .
I am running twin pipes and had to mod the cross-member to suit .
just an idea , not sure about your lower seat issue will affect it.