I have an A518 in my '73 Dart.
Top side of the crossmember needs to be removed and rebuilt/replaced with either a thick plate or on the inside of the cabin. (If you don't replace the crossmember top, the crossmember left&right will start to tear loose from the floorboards. Ask me how I know...

Lower crossmember can be modified to accept the A518 mount. A mid '80s GM mounts works nicely.
On the left and right side of the transmission tailhousing I trimmed the strengthening ribs for clearance.
I 'massaged' the tunnel near the OD-flange to the transmission body to make the transmission sit at a good/better angle. Otherwise the (stock) deep oilpan will sit lower and driveline angle is worse.

I have some pics on my website but not really the mods needed to the transmission or tunnel.