M1 Pistol Grip Gap Question

[Nitro_Kid]

I recently swapped the conventional stock for a pistol grip stock on my M1. There seems to be a pretty significant gap between the back of the receiver and the front of the stock - maybe a tenth of an inch or so; much larger than the slight gap visible with the factory stock.

 

Any suggestions on how to reduce or fill the gap?

 

Background: I bought the pistol grip stock used, and it looks like a correct factory benelli stock, but could be earlier or later production than my gun.