Its very common to have the gap you are talking about. How critical it is I guess depends on how critical a person you are. I'm certain an aweful lot of Marlins serve well like that for years. BUT, IMO its a critical area, especially in the harder recoiling guns. Ideally thats where recoil forces are transmitted from the action to the stock. With a gap there the action can set back with recoil & since the tangs are tapered they are effectively a wedge if the action can slip back.
If theres room behind the tangs its not very hard to correct. Put some inleting black (or the wifes lipstick) on the reciever and tangs where they meet the wood & then push the stock on. The inletting black should have marked the wood whever its making contact. Be VERY careful removing wood from the tang inletting as its easy to go to far & you cant go back. Useing a file erase the marks & try again. Keep going until its nice & tight against the reciever.
Alternately you could just use brown died acraglass to fill the void. I generally get it as close as my patience allows & then bed the reciever & stock together. That way you get a purfect fit with no gaps or wobbles. I dont like seeing the glass & thats why I bother with the fitting first. If you get it very close you wont see the bedding. If you do bed it make sure you use release agent. I usually coat everything several times, includeing around the flare & tangs, not just what will abut the metal but on the outside as well. Reason being is if some glass squeezes out, and it most likely will, I can just wipe if off & not have it smeared on the metal. Take a look in the reference library for better descriptions of stock fitting & bedding.
