I would say you can make your own shims from steel or brass without an issue. My belief is Reese is more concerned about the amount of stress put on the clevis part of the head during tightening then if it is their shim or your home made one. Ask them when you call.
By their instructions, it appears they allow 1/2 of their shim thickness (air space) Or 0.021" of gap that the clevis can compress. A lot over that 0.021" and issues may start.
Your concerns about clamping up the head with no shims, this may be better asked, what was the gap?
If you want to have a level of confidence you did not create an issue I know of 2 methods to look for cracks that could tell you if something started.
1. Wet fluorescent magnetic partial inspection. (Magnaflux). Have to find a shop with a big enough magnet to put the head in.
2. Dye Penetrant inspection. Most weld shops have this, are trained and does a good job on cracks wide enough for the penetrant to get into.
Cost. You are up against the cost of the entire hitch head. This price is accurate however that pic is the older 3 piece head.
http://www.etrailer.com/Accessories-and-Parts/Reese/RP58167.htmlIf you have a buddy who is in a weld shop or in a NDT lab, they can do a crack test labor free and only the cost of materials. If you ask for a quote and they quote 1 hour of time, an all new hitch head may be cheaper.
I know the question swirling around in your head, how does one know you created a problem or not??? Good question. There use to be pic's of the cracked heads here on RV.net but last I looked they did not show up, OP may have moved his photo server. Not a pleasant site to see you hitch head cracked in 1/2.
Hope this helps
John
2005 Ford F350 Super Duty, 4x4; 6.8L V10 with 4.10 RA, 21,000 GCWR, 11,000 GVWR, upgraded 2 1/2" Towbeast Receiver. Hitched with a 1,700# Reese HP WD, HP Dual Cam to a 2004 Sunline Solaris T310R travel trailer.