7.5 shot is all you need. I usually use a 3&1/4 dram behind 1&1/8 oz 7.5's & kill crows very consistantly. I've used anything from BB's to 9 shot & the 7.5's with a light mod/modified choke work best for all around. I've even lucked up & killed a few ~60yards with the 7.5's on hail mary shots with the light loads.
If you were wanting the best, you could go 3&3/4 dram of 1& 1/4 oz 7.5 shot, 6's at the largest (although 4's can work well with a tight enough choke, your just giving up a lot). Most guys will kill MANY more with 8's & an IC than #4's & a full.
I don't hate crows enough to shoot a 3" shell at them. The cost, nor do I want to take the punishment that comes with it. We shoot an average of 70-80 shots a day of run & gun method (do that with a heavy 3" load). Learn how to get hide good (get in the thick stuff, stay out of the sun & don't be moving when they respond). If the place has been over called that bad find a new area (this is the best advice). It's nearly impossible to kill overcalled crows with a shotgun (I'm not talking 1 or 2, I want to knock down 10-20 per stand or it's not worth the effort). Also, I don't use decoys much at all either. Again an effort thing & they can also buger the crows(if you do use decoys flock them & amke SURE they are out of the sun).
Simply put; you will no kill crows consistantly @ 60 yards with a shotgun; no matter what your shooting at them (well maybe a punt gun). Put up a pie-plate at 60 yards & shoot it sitting still. I'd be willing to bet you'll get a small # of pellets on the plate (I'm talking single digits if your shooting large shot). Now imagine if that plate is much smaller, moving 40mph, & those few pellets that did hit the plate, are spaced out into a 7-8 ft shot string.
Even if you did find a true 60 yard load (& more importantly; could use it), one trip of that & they will learn quick to stay 80-100 yards away. Actually most "overcalled" crows will go the other way when you turn on the call (or just sit tight). I'd try getting hid better & find new spots. Even if your still end up calling the same area, make sure to go to a different spot on the property. They remember were sam, tommy, mark, lakisha, justin, jenny, karen, & tyron went down last time!