Ugh, in the new version the Pak40 and Zis-3 fit, the QF6 did not (who would have guessed that that's the widest...) Updated now so all of them will go through that gap.
Is there a list somewhere of how big a path needs to be for units to pass thru?
Quick testing showed these crazy results:
EDIT (7/24):
HOLE SIZE NEEDED TO PASS THRU
1 METER
OST Pak40
SOV ZiS-3
OKW Rak 43 
3 METER
SOVIET M42   
USF M1 57mm  
BRIT QF6     
Thats right, the tiny M42 needs a bigger hole than a ZiS or Pak40