Engine 302 oil leak when in gear but fine at idle

[AlexM556]

I have a 67 mustang and I recently changed the oil and filter and also replaced the gasket, pan, and rear main seal on my 302 since it has had a slight leak for a while and I wanted to finally fix it but it seems I created another problem. I drained a bit less than 5 quarts but when I filled it up, it only took 4.5 quarts before the dipstick shows oil almost at the "max overfill" line which is odd especially since the new pan is the same capacity as the original if not slightly more. I let it idle for a while but the oil level stayed the same. There was no sign of a leak and everything sounded fine so I didnt think much of it. I got in to take it for a short test drive and it reversed out of the garage no problem. I put it in drive and made it about 5 feet before smoke came up from under the hood. Oil was leaking on the driver's side somewhere and it dripped down onto the exhaust header and caught fire. I was able to put it out before it did any damage as far as I can tell but I have no idea as to what the problem is. The entire driver side of the engine bay now has a nice coat of new oil on it and the leak stopped after I shut off the engine so I never saw where it was coming from. My guess is that the breather/filler cap on the driver side valve cover started to purge oil but I thought that only happened when it is severely overfilled. Anyone know what to do? Thanks!