With out the pins joining the two plate, you have NO way to keep the front portion of the forestock from turning around the barrel, or pulling away from the joint of the two plates. Not Good. The pins there are intended to help hold the two-piece stock together. Follow the directions.
The one thing I see poorly done with these stocks is that the kit builders fail to taper the forestock from the entry pipe to the muzzle cap, on both the TOP and bottom of the stock. You end up with a very square, clunky looking, and feeling forestock, with a ramrod hole that is way too deep, and a boring straight line the full length of the top of the stock from breech to muzzle. At the breech, the top line of the stock at the barrel mortise should cover about 2/3 of the bottom of the side flat. But, at the muzzle, that stock should cover 1/4-1/3 of the side flat from the bottom. The stock should look like its merely there to offer guidance to the barrel, rather than support!
The stock's only real purpose is to allow a place to hang the ramrod, and to protect the barrel from dents, dings, and being bent in any fall. Let the wood forestock take the damage, as it can be easier replaced.
The STOCK- neither part-- is supporting the barrel. Instead, on MLers, the forestock HANGS off the barrel. The thinner the forestock is towards the muzzle, the better the balance, and the less weight the barrel has to carry.