I need a 1955 Remington 870 receiver to 3.0" Magnum

[LowSpeed_HighDrag]

I have seen some used Police 870 for $275.00 - $345.00 I can buy them used cheap at my Dealer cost.

That is probably the best route. I just bought a lightly used 870P from my local police supply store for $230.

[Don Robison]

Any competent gunsmith can do it. All that is involved is changing the ejector from the 2.75" one to the 3" one. It will require refinish unless you don't mind the mismatched rivets. If you don't plan on shooting 3" shells you can still use the barrel and just shoot 2.75 shells out of it. My 14" 870 is setup this way. I have no intention of shooting 3" shells so I didn't convert it over to a 3" ejector.