2 small tricks. Slightly loosen the allen screws on the marker, only the rear half of the marker tho - every allen bolt from the tombstone back to the stock. This should make the stock fit less tight, enough for you to get it on easily. When it's in, tighten the bolts.

Secondly, put some white lithium grease or gun oil on those 2 rear push pins that hold the stock in place. The oil will help them slide in easier.

Without your barrel on, place the marker on a carpeted floor with the marker pointed into the carpet and the rear end facing up. place the stock in, and push down, the weight pushing down on the stock will make it down as far as it will go. Slide in the pins, and you're done.