518 in an A body

I did a 727 to a518 swap in a E-body (73 challenger). There is a lot more room there than in A-body. There is a nice write up on the web about this(727 to A518 swap – a novice guide). It may be useful for small details after you do the installation - controls, piping etc.
One idea is maybe looking at a A500 unit instead of A518. It is smaller, based on 904, lower first gear. Have one in my 91 D150 and its a good running transmission.
Also make sure you get the hydraulic version of these units, not the electronically/computer controlled later units. The electrical connector has to be with 2 or 3 pins, no more.
I am happy with the A518 and 340 in my challenger. Nice hwy cruiser.