You might be able to get away with the stock size, but I don't have mine mated up to an aluminum bell housing so I can't say for sure. I've got mine mated to a Mcleod bellhousing which required me to drill out the 4 holes on the tranny to half an inch. I think on the aluminum bell you could probably use stock size though and not have to drill the trans.
It has been a while since I checked but factory bellhousing to transmission bolts are metric threads.
The Tremec 3550 requires a different dedicated bellhousing and uses SAE bolts for attachment. If memory serves you will need 4 1 1/2"long X 7/16 X 14tpi bolts. Don't take the thread pitch as gospel you may need 7/16X20tpi. check that first.
Called D&D today and they are different. The Tremec takes a 7/16 course thread bolt and they don't sell them seperatly. They told me to go to a hardware store and get them.