Need some help !!!!!!!!!!!!!!!!!!!!!!!!!

As I understand the OP's situation, this engine only has a few miles on it. It has been mentioned before that this is not really broken in. The cam may be, but the rings are not likely anywhere near seated IMHO. It needs some driving and loading the rings, once any leaks are resolved.

We don't know HOW much the engine had cooled when the OP released the rad cap and found some unknown amount of pressure behind it. OP, was the engine stone cold, like having set overnight, when you removed the cap and found the residual pressure, or just cooled down for an hour or a few? And, if the engine has not been heat cycled enough to get the trapped air all out, then it will tend to have more residual pressure until that is done. OP, do you have an overflow tank on this setup?

The leaks under the t'stat housing simply don't look like fresh oil; no way it will be that black after a few miles. Likely rust or sealant like MT mentioned.

The low end miss? Could be quite a number of things, like carb tuning or ignition. Or simply just not being used to the exhaust note.

So more detailed info is needed from the OP on several items.