There are 2 ways to go here. The first is to fit 2 stock sets. One for offhand and one for benchrest. 
The second is to use a traditional stock that will do a fair job at both. For this choice I would reccomend a CPA in either #52 or #49 stock set. My other choice would be a Union Hill Ballard.

40 Rod

In ASSRA matches it is legal to use a combination butt plate. You only have to change the buttplate to go from bench to offhand. I am talking about the Freeland buttplates used on the
BSA Internationals. Freeland's are no longer available new but there are others that are. Anschutz combination plates work extremely well but they are VERY pricey. There are others - you might check with Brownell's. 

If you get the proper kind, you can also adjust length of pull as well as go from flat to hook. We have used them for years in position small bore shooting. They are not "traditional" but neither is shooting both bench and offhand (most, if not all, of the traditional matches were all offhand). This may not be the most elegant situation but it is certainly the most practical.

The CPA problem (loose tang screws) can be solved by installing machine screw bases permanently in the stock under the tangs. They do not "wear out". Bernie Harrell does this regularly. He is definitely a "Stevens Man" and has been using this method for years.

Dale53

Well,if it were up to me,I believe I would go with a pistol grip style stock and maybe a cresent butt..And I think as far as caliber.I would go with the 38-55..My reason being that it is less effected by the wind and a little bit easier to get a good load as long as the bullet match's the twist..I may be wrong here,but I think I would go with something like a 15 or 14 twist..The 18 is too slow for most bullets..But there are no doubt some shooters here,that have a lot more experince than I do..So you take it from here..And do with it what you want..Good luck..jp

MartiniBelgian;
Yes, you are correct. We are talking about machine screw "inserts". The ones I have used have extremely coarse threads on the outside and machine screw threads on the inside. I have not watched Bernie install these but he told me that he epoxies the inserts in the wood. 

Second question: this way would be better than wood screws to hold the rear sight. However, neither is the way I would do it. I would make up an "extension tang" drilled and tapped for the sight screws and carefully inlet that into the top of the wrist and expoxy it in. Ideally, would be to have the extension tang welded to the rear of the action, if possible. However, even if it was not welded to the action it would be much more stable and secure than just two screws. I have seen many installations like this and the people who owned them had full confidence in the procedure after long time use.

Dale53

Good morning, Mr. Belgian;

I have a stock that I am putting in a sight base just like Glenn said.  When I ordered it, I had them inlet the stock exactly like the action had a tang on it.  But I had them leave about 1/8 inch (three or four millimeters) of wood behind the action.  I intend to inlet a block of steel into the wood.  I left the wood wall so the steel sight base wouldn't touch the rear of the receiver.

Y'all be good.

horsefly

Thanks Gentlemen,

I get the picture - Maybe I should have that done indeed... - but I won't do the inletting myself though!