The 307 has about a 1/4" shorter stroke than the 350 which translates to less torque, so you'll need to wind it up a little tighter to make it move that big rig effectively. The 600 CFM carb is plenty big to flow enough air to get the job done. If you can find a tall-block 427, I think you'd really like what it can do for low-end pulling power. I'd pick that one over just about any way a 350 can be set up for load-pulling use.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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