As was said before, you absolutely MUST remove that powdercoat from your mounting surfaces. This is not debatable. You will have much bigger problems with stress fractures in your engine and/or tranny down the road if they are not properly shimmed and mounted - metal to metal, you should shoot for less than .001" gap between your mating surfaces, but some will argue that .005 is acceptable (but not recommended). There is a proper procedure to check this:
[I'm assuming you're running either an enclosed primary or an open belt with a motorplate.]
1. with your rear engine mount bolts torqued to half spec - measure for front shims with a feeler gauge and shim accordingly
2. then install your primary to your engine and transmission, you won't need to use all of the bolts (although it's not a bad idea), the point here is that you want a solid unit. You also want to make sure that you have zero gap between your primary and engine, and your primary and trans..you may have to loosen up your rear motor mounts to do this, this is very important.
[edit] 2a. Retorque your motor mounts.
3. Measure for shims on the transmission (i've seen up to 1/16" gaps on these so be prepared to shim the dickens out of them if necessary.
4. Now with your solid unit and shims there should be no gaps in your mounting surfaces. Some claim that you can get away with .005" but I wouldn't recommend it because .005 is super easy to shim anyway and a 50 cent part from mcmastercarr is cheap insurance compared to the cost of a new engine case.
Make sure you are 100% clear on this process.
That said, most powdercoating processes only add a few thousandths of an inch to the thickness of the part or up to 1/16" (depending on the powdercoater and process used). This will not account for a 1/2" of missing belt slack. It was said before that new belts are tighter, these are designed to stretch out over time. One thing you can do is soak the belt in hot water for a few minutes, this will cause the belt to slightly expand which may give you the extra slack you need to install it.
Good luck
