Where to purchase a A-500 / A-518
If you can it would be easier to do an A518. While finding a A500 core and a good builder is a challenge, I found it's much more difficult to find a decent converter without spending $1000. Sure, Summit sells Hughes lockup converters for the A500, but if you need more than 2500 stall you're pretty much boned. The A518 on the other hand is much easier overall, and can be built non lockup to run any 727 converer. In my case the 518 was just too big so it wouldn't work.
On a side note, I payed a visit to Phoenix Transmission (cool place) trying to buy one of their A500s and the owner flat out talked me out of it. While I was there they had a nice Lil Red Express on the lift that was running a A518 and was swapping to a built 4L60E.