I put a 518 in my 77 CrewCab & I can promise you dealing with a reg trans tunnel will be harder to do than mine was.
I cut everything that was extra off the outside of the case to help make it fit the only saving grace was the long floor was able to be jacked up to clear
I getting ready to put a 93 firewall & floor with trans tunnel in it

Use the crossmember for the 518 in a 90-93 so the driveshaft angle stays correct

I'll see if I can find the pics of everything that was cut off & hammered to fit the early A/T trans tunnel