Ring end gap too narrow for boost?
I'd set the Whipple aside for a mild 350. As already mentioned, you can't spin it fast enough for a 383 or 406.
[opening a can of worms]
As far as the whole SBC vs LS debate, repeat after me: "Power under the curve. Power under the curve. Power under the curve." ...and then apply that to what rpm band you're going to be driving in. He's already said "I rarely drive over 4500rpm". Give it up.
There's no way I'd do an LS swap for a tow vehicle. One of the comments was "Yes, I know that peak tq comes on earlier in the big block, but you're only going to notice that pulling max weight. Which if you were serious about, you'd have a diesel anyway...."
Wow, so we should either suffer with an LS motor that can't get the load moving from a stop, or buy a diesel that's twice the cost? No thanks, bruh. I'll stick with a well-built small block or a mild big block that is right in the middle of both capabilities.