I have a Comp Cams Extreme Energy XE274 (one step above the 268 cam mentioned above) in a 406 sbc in my Crosley. Has run as low as 10.30's in a 2,300 lb. car. If you put too much cam in you kill low and mid-range power. The more displacement the engine has the more cam you can use. I believe the Thumper series uses more duration to give it a lumpy idle. Above idle it functions like any other after market cam.
I was looking at dyno results on the Comp Cams site. In a 358 sbc the 268 cam made more horsepower than the 274 cam at 2,500 rpm. The 274 cam didn't pass the smaller cam till about 5,000 rpm.
Picking a camshaft is both exciting and confusing. Have fun.