I'd be fearful of running an engine with .045" clearance, especially if it were the exhaust valve which gets hotter an expands more. I've gone by the basic rule of .080" on the intake and .100" on the exhaust, but on occasion have run them slightly tighter than that. When it comes to advancing or retarding the camshaft, it depends on which valve you're having insufficient clearance on. In most problematic cases the the piston is chasing the opened intake valve and hits it before it can fully shut or hits it as the valve "bounces shut". To help reduce the risk of this situation you can retard the camshaft some and it will provide additional intake valve clearance as well as reduce your dynamic compression slightly. If your exhaust valve is the concern, retarding will only make that situation worse, and you'd want to advance the camshaft a few degrees to pick up some extra clearance on the exhaust valve. So you really need to know how much clearance you have on both valves before knowing if you have the room to advance or retard the cam -it's not always an option as it's a bit of a balancing act.

Different head gaskets vs optimal quench have been discussed here already.

You can also choose a camshaft that has a different intake closing or exhaust opening point to help improve your situation.

-Dan