Sounds like the most likely problem is a blown gasket or head. You can get a compression tester at your local auto parts place for about $30-$40 and test it yourself. Just pull out all of your plugs, screw in the compression tester and crank a few revolutions at each spark plug port. The readings should all be within 20% of eachother, if it is more you either have a blown head gasket or warped head, block, or worse a crack! If it looks like it's just the head gasket, have the head checked for warpage or, for about $120 in tools you can test it yourself. Go to
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and buy a 24" precision straightedge and set of feeler guages starting from .002" and larger. Since you've done the timing belt yourself, I'll assume you have a decent shop manual. It will show you how to check the head and block for warpage. If the head gasket looks ok, I'd think about swapping the long block as that means there's deeper problems. Between the overheating and excess oil added, these are sure to have done some damage.