I put a 518 behind the Hemi last year. There is room for the 518s and there are plenty of them. The cores cost about the same and there is not a lot of money difference to rebuild each, so there is not much incentive for you to go with the lighter 500.

I did not have to do any crossmember cutting putting one into my 70 Cuda. There was one place that needed a 2 inch or so dent make in the floor board for clearance. I bought a fabbed crossmember, but they are not complicated to make. I am just not good at fabrication, so buying one already made was easier.

You will likely have the thing rebuilt before you put it in, so which one you start with (46, 47, 48) won't matter much in the end. As long as it's an RH (not RE) which had the small oval 2 or 3 pin connector.

The earliest 518s had the 2 pin connector. After a couple of years, they added the lock-up torque converter and the connector became a 3 pin to accommodate it.

I went with the non lock-up because there is a much better selection of torque converters. But for a milder street car, that might not be an issue and there can certainly be an argument made for the lockup. And they made the lock-up version longer, so there actually should be more of them.

If you have a competent 727 tranny guy locally, I feel the best bet is to grab a core at a salvage yard ($200+/-) and have it built the way you want it. That can be anywhere from $600 to $1,200.

Based on my experience, the cost of the swap could be anywhere from $1,000 to $2,500.

There are a number of threads around that detail the swap. One has step by step pictures. It is bit involved, but not too bad.