So I'm looking into getting a Carvin fretless neck for my bass, a Squier MB-5. The specs match up...34" and 22 frets...but I'm not sure if squier uses the same bolt pattern as the J/P basses on. (It's a squier-only body shape, not a P/J copy)

I'm assuming that it would work, but it's always better to check..
Nope, no sig here.
from what i heard the neck pocket on a carvin is smaller (in depth and width) than a fender, but this is for the 4 string necks. I heard this from a guy selling a carvin neck.
Ibanez RGT42DXFX with emg 81/85
Schecter C1-Classic
Vox AD30VT
Knock off P-Bass
I tried to put a carvin neck on my fender. The neck heel is a bit wider then a fender so would need to be sanded down to fit.