First of all, welcome to the forum. This is a great place will lots of friendly and helpful folks.
You have a fine turkey gun in your Winchester 1300. They are becoming hard to find and are quite sought after by many shooters and hunters.
I'd recommend that before you pattern your gun, you thoroughly "deep clean" the barrel. Sometimes barrels, regardless of age, have never really had a thorough cleaning and it can make a great deal of difference in performance. If you'll look at http://www.allaboutshooting.com
for the article "Cleaning a Shotgun Barrel"
there are some guidelines that I believe you'll find to be helpful. It's not rocket science but it just works.
With your 24" barrel, I'd recommend that you try the 3" Winchester HV (Black Shells) with 1.75 oz. of #6 shot. That's a proven load, widely available, reasonably priced and has probably been responsible for more turkey kills than any other turkey shotshell on the market today.
Lastly, I'd recommend the SSX Turkey Choke. It has great internal geometry, is rated for all lead and "hard" (tungsten-based) shot and is very reasonably priced for a premium quality turkey choke.
If I can be of help, just let me know and I'll do all that I can to be of assistance.