I've replaced quite a few stocks and if you go with a factory stock it should be pretty close to a drop in fit. It is not unusual to have to use a little sandpaper or a dremel tool to remove a little stock material to get a good fit. It seems giving the safety lever room to move is the most common problem I run into.
The barrels should be the same since 1975, but Remington is using a differrent trigger and the safety is a little different on newer guns so there may be a little work around those areas. Probably not though.
If I were replacing the stock I'd look for a better replacement than the cheap factory plastic. There are fewer options for the ADL, but I'd use this opportunity to get a quality stock from one of the better stockmakers such as McMillan, High-Tech, MPI, Brown Precision, etc. Bell & Carlson makes a good stock in their Medalist line at around $200, but do not offer the Medalist in ADL. The only stock they offer in ADL is meant for "professional use only". And they mean it. I've done 1 of those and it is far, far from a drop in. And not anywhere near as good as the Medalist.
If your wood stock is just beat up you can often find great deals on very good used stocks on E-Bay and places such as Gunbroker. I've bought most of my McMillans from guys in classified ads on gun forums. Lots of guys replace the factory stocks with aftermarket and then sell the almost new take-off's. Or decide to sell a rifle, put the factory stock back on it and sell the aftermarket stock used. If you're patient you can find deals. I bought a rifle in June and have been waiting on a deal since then. Placed the order today.
www.mcmfamily.com offers specials fairly often. When someone orders from them they place a deposit on their stock. If for some reason they decide not to finish paying it off McMillan posts it on their website and sells it for the amount needed to pay off the stock. I just ordered one of those today. Paid $375 for a stock that would normally sell for $600 and take 3-4 months to get. I'll have mine in a few days. It is in a color I wouldn't have picked, but to save $225 I'll either live with it, or paint it.
