The Model 21 mystique has gotten more than a few shotgunners in trouble.
You asked for advice and I am going to give it. Buy only from a truly reputable dealer with years of experience in Model 21s. Just because a dealer has a lot of Model 21s in his inventory does not make him reputable!
Like in Real Estate where location is all that matters, Originality of Condition is all that matters with 21s. If a M-21 has all of its original parts and dimensions and has been re-blued and had its wood refinished, it is still a Winchester but it has much less value than an all original gun. PROBLEM:not all novices are skilled in knowing the difference. Let me put it this way...a restored or reconfigured or reproduced M-21 is VERY, VERY easy to buy(there is one behind every tree)but they are VERY, VERY HARD to sell. A really nice all original M-21 has its own intrinsic value, dollar amount notwithstanding. It is arguably the finest shotgun ever built. Owning a clone may be OK for some folks, but it will make you miserable in a very short length of time.
M-21s were, in many cases, built to order and because of that you should be able to find one with the dimensions that will meet your needs, but that factor is going to make your search more exhausting. PROBLEM:if you buy an all original M-21 that doesn't fit and you have a custom buttstock installed, you will have destroyed both the intrinsic value and the original condition value.
From my experience, a Winchester Model-21 that fits and is in excellent original condition is a priceless shotgun and a thing of beauty that will give a lifetime's pleasure.
Kit