The 59 Roundback looks like your best bet.
Remember to get a 25.5" scale length, and that the tenon fits the pocket.
^ Really? I would think the Fat neck profile would be closer to a Gibson. Anyone else want to chime in?
"teh most gains"
The 'fat' neck is much thicker than any current Gibson neck. It's even a little thicker than the original necks from the 50's. The 'fat' neck is a straight 1" thick all the way down, Gibson have never made a neck that thick in production. Even the '59 neck profile is slightly thicker than any of the modern Gibson neck profiles.

Quote by Baby Joel

Remember to get a 25.5" scale length,
Or if he wants the Gibson feel, he can get one of the 24.75" conversion necks. Botht he 25.5" scale and the 24.75" scale Warmoth necks are designed to fit on 25.5" scale bodies.

and that the tenon fits the pocket.
This is the problem you're going to have, I do not believe the Warmoth necks will fit a Squier body. The Squier is made to import (metric) measurements while the Warmoth is made to the US (imperial) measurements. I doubt it will fit.
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^^^The warmoth neck drops in a squire with no issues. I had one of the conversion necks in mine for a while.