I'm also running a 5.3L and I just put an Isky 224/233 at 112 LSA in it. A 3K stall isn't going to be enough for it. I'm going to have to run a 3500 to get the rpm's where I need them... this may be the case for your thumper, too.

I haven't had it tuned or dyno'd yet, I'll be posting numbers when I do.