for an uppercut to the body, just tuck your elbows and twist a little. for overhands, just keep your hands up and move. those are probably two of the easiest punches to counter. I've never had an uppercut thrown to my body, but I would probably counter with a hook.
in infighting....since you're moving, pushing, etc., uppercuts to the stomach will be fairly easy to block, since you can just turn and angle. For the overhand right, you just have to keep your hands up and/or duck or move out of the way. And sometimes you just have to take a punch.
If you drop your arms a bit and roll (twist), you can block em, but that leaves your face exposed. Better way is to keep your hands up and just bend the knees so you lower your whole body and catch the shots on the elbows and forearms. Your face is still protected and any follow up shots intended for the head will probably miss because you alrerady dipped under.
For the overhands, bending at the knees also is one of the best ways to slip them. It also allows you to roll under the overhand too. Whatever you do, don't pull away from an overhand.
Always be prepared to move ... and if your not quick enough your hands should always be there to protect the overhand right.
Tuck your elbows in and twist your hips. You'll hit the punch off track or block it. You can also parry uppercuts to head and body. Lastly you can pull your head and body back using foot and head movement. However this isn't the first thing you'd advise.