I recently aquired a Faxon barrel that seems like it is designed for a low pro gas block. My gas block is a clamp on flip up front sight type by YHM. There is a tiny "step" built into the barrel that concerns me. The thickness changes at that step but is extremely tiny. I am just wondering if it will be an issue. It fits, clamps well, and seems secure.  http://imgur.com/LiTDAXY

I would try a shim under the gasblock.

I don't see any problem with it, as long as the gas block is lined up with the hole in the barrel it will shoot fine, as far as a shim, I don't think there is enough of a difference to cause problems with it being sighted in.  Go out and shoot it and see what you got before being concerned about it, Faxon knows what they are doing.

I'd imagine that barrel is profiled to be used with a LPGB, but that small step shouldn't cause any issues.

Testing it out will be the only way to know for sure, though.

I am talking about that line that is between the clamps of my gas block in the picture. It is just a hair difference in diameter. I am just trying to find out if that looks like it will be an issue.

http://imgur.com/LiTDAXY

The OD of the GB journal should be at least slightly larger (e.g., .750")  than the OD of the section forward to the muzzle (e.g., .740").  That way, a set screw low pro or a GI-type sight base can easily slide over the muzzle and still fit tightly on the journal.  

Most barrel makers leave the GB journal long enough to accommodate both short lo pros and the longer GI style such as the YHM model the OP is using, other makers just assume only short GBs will be installed.  Same with the distance from the gas port to the shoulder behind it, some barrels are drilled short for GBs used with FF handguards, some are drilled .020" longer to accommodate the front end cap required for clamp on type handguards like Magpul's.  

Ideally, barrel makers are perfectly clear in their product descriptions so you know in advance if the barrel is going to properly fit your gas block and handguard. - CW

This.

Assuming the "step" is 0.010 inch high (0.020" diametrical change), your clamp-on gas block is sitting in a rear-high attitude of +0.3 degrees.

Worst case, assuming you have a mid-length barrel, the rear end of your gas tube is now 0.05 inch high, so it will have to bend slightly to pass through the hole in the upper by that amount, but then the large bearing surface of the rear clamp will cause the gas block to bend so the gas tube sits more-or-less straight.

I wouldn't worry about it.  Although, I would torque the rear screws first.

Is there any high temp sealant that can be used SPARINGLY between the gas block and the barrel. I also have some small leaks that have not sealed from dirt or carbon. I am also using a Low-pro block with 2 set screws.