I have a 73 cuda 340 with an A500 which is the 904 version of the 727/518. I love the lower first gear and the OD which is simply turned on and off when in high range via a toggle switch. It is dimensionally the same as the 518 at the case to tailshaft overdrive split and rearward, and thus posed the same challenges at clearing the crossmember that the 518 will. I did not want to butcher my crossmember, but took a cutting wheel/grinder to clearance the tranny itself as much as I could stand, especially at the case to OD flange where there are pipethread pressure test plugs. I also "bulged" my floorpan and dented the upper crossmember a bit in a couple places where interference was still a problem. You can cut the lower crossmember cradle to weld in a new rear tranny mount system, or try and buy a GearVendors crossmember cradle and mount which is what I eventually did and run to this day. Did this mod in the late "90s and still using it today tho with a non lockup 904 hi stall converter and modified (additional clutch plate) front clutch and front pump system from a 904. I always wanted to do this with a 518 but could not find one and convinced myself the lower first gear ratio of the A500 was more suitable for getting my somewhat heavy Cuda out of the starting gate. I did shorten the driveshaft by about 2.75 inches as I recall and had to mess with drivelshaft angles a bit to prevent an annoying vibration. Love it and it was a worthwhile mod. Now I want an 8 speed!!!!