Alot of balancers do not fit correct and will end up either hitting the front cover at the timing marks before it is seated or they are too think and you need to space your accessories. I have used the Rattler before and it fit perfect.
Personally I've never cared for the Fluidamper. The conventional type has and continues to work well for many companies. I've been using the SFI neutral balanced balancer/damper from Pioneer #872037 on my internally balanced 360's for years. No problems and is a quality, reasonably priced race unit.
http://www.summitracing.com/parts/pio-872037
Disregard the pic Summit has. It's generic and the Pioneer looks nothing like it.
I guess I didn't worry about the timing cover.
I did have an ATI on the 340 that was in the car before and the pulleys did not line up well. I had to move the alternator and water pump pulleys out around .125 if I recall.