I thought all the M4 designations were large pin, except Colt's? I don't have enough information... ...
As for the A1 and A2 uppers, both have the carry handle. The original M16 upper was a slick side. No brass deflector and no forward assist. The rear sight was adjustable for windage only.
The A1 upper originally had a forward assist and no brass deflector. The rear sight was adjustable for windage only.
The later A1 uppers, also known as the C7, had the brass deflector and a forward assist. The rear sight was adjustable for windage only.
The A2 has both, the brass deflector, and the forward assist. The rear sight is adjustable for windage and elevation.
As far as converting it to a gas piston, it depends on what kit you want. There are several manufacturers out there for direct impingement uppers and kits.
I've used the CMMG kits on several rifles with good results. I like direct impingement, personally, but I'm not sure how much of an upgrade it
really is.
CMMG kits:
http://www.midwestindustriesinc.com/index.cfm?fuseaction=category.display&category_id=352
Osprey Defense:
http://www.gaspiston.com/about-2/why-gas-piston/
Adams Arms:
http://adamsarms.net/products.asp?category=100000
Oh, and for your shims, what do you mean? Do you have side shims that go on each side of the upper, around the forward attachment pin? Or are you talking about the tension pins? OR are you talking about forward attachment pin bushings??!?!?
