Small block? Pfft. 454 with a hot cam in it is more like it.
Buddy used to have a truck with a done up Ford 460 in it. Sounded surprisingly similar to my bike with Holeshots. (just it was a LOT louder, haha).
Dale Walker (Holeshot Performance) doesn't make the slip-ons any more, just full systems.
To my knowledge Supertrapps are the only slip-ons that are still being made new and are fairly popular. You can play with the # of discs to make it as loud or quiet as you want. You may find some used cans of other brands. Cobra 4-4's should be avoided, they cause a big HP loss from too little backpressure.
The supertrapps sound fairly similar to the Holeshots, they have a bit more "boomy" bass to them, a bit more "woof woof woof woof" underneath. Both sounded great.