Piston Head clearance for the 300 is the same as a 250.
In my experience with 3 Wilf Green 300s all had wide squish bands 1.75 to 3mm,
larger the clearance the lower down the rev range the engine pulled from, but less top end, 3mm 65mph.
I had them machined down to 1mm clearance and the engine had the same power characteristic’s as a 250 but with more GO!!