It would be cheaper to buy a barrle that is already set up for choke tubes. You might use 3.5 inch gun for crows, once, and then never want to do it again. Ive killed hundreds with a 2 3/4 inch remmy, and the eye surgens never
had to re attach my retinas.
Check Gunbroker, used 870 barrles are everywhere.

 

Just get an 870 barrel threaded for choke tubes if you want some versatility. As for 3 1/2" chamber needs, you don't need this type of firepower for crows. They'll end up having the last laugh as you'll be in the poor house buying expensive shells.

Nothing wrong with a 2 3/4" chambered 12 ga. or a fixed-modified choke either. That combination can probably serve you well for most situations for crow with the right loads.

I was out varmint hunting in the snowstorm Friday, and managed to call some birds in and connected with one. All I used was an old 16 ga. w/ fixed modified choke:

 

How far was that shot setter? The only reason why i was asking about a 3 1/2 inch is because the crows around here really hang out.. like 45+ yards

 

I would guess 40 yards. He was incoming above the treetops in a mature forest and out a ways from me. (He was pretty small-looking when I passed the bead through him.) I was using a duplex reload I concoct myself; 3/8 oz. of #6's over 3/4 oz. over #7 1/2's. Total payload is 1 1/8 oz. and it's moving at only 1200fps.

Remember, it's not the size of the shell that reaches out at any distance, but the size of the shot you are using and the energy it has at that distance.

I do prefer #6's and #5's for crow and have seen #5's reach out pretty far to fold birds. There's not much to a crow, they are pretty frail when it comes to bone structure yet they represent a small target.

I kill a few crows each year with a 20 ga. shooting 1 oz. reloads with #6 & #5 shot out of a modified fixed-choke A5. You don't need much, just the right loads.

 

I had an 870 redone for my grandson when he first started to hunt, the gunsmith threaded the barrel and gave me several chokes for it he also lengthen the forcing cones and it now shoots 3" very well did all this and shortened the stalk and it only set me back about 100.00. He did an excellent job and the gun has given my grandson many years of service. No problems with changing chokes etc.

 

Any competent gunsmith should be able to install the interchangeable choke system of your choice. There is plenty of steel in Remington barrels for this. However, personally I would leave it alone if you want to shoot crows with it. Crows are normally straight overhead and have their wings spread, making for a large target with kill zones and wing bones open. 2 3/4 inch shells are fine for them. You most certainly do NOT need 3 1/2 inch shells for crow hunting.

 

To reach out a little bit more, go to a shot size larger like a #5 or even #4 in an 1 1/4 oz or even 1 1/2 oz load. Shooting $18.00 or more loads for crows makes no sense.