Why am I leaking oil...grrrr

I have a freshly rebuilt 383 in my dart. I replaced the rear main correctly but have a leak especially when rpms are up. I pulled the oil pan and replaced the main again and let it cure over the weekend. Started it today and let it idle for about 10 minutes at about 160 degrees.. No leaks...I drove it for 10 miles at about a max of 3500 rpms..it poured oil and looked like a smoke cloud behind me. I parked in my driveway and it left an oil slick the size of a sheet of paper. After it cooled, I started it and ran it again at idle for about 15 minutes WITHOUT any leaks...what the hell is going on? I have an oil catch can coming off the valve covers and the top is releasing pressure (smoke mist coming out of the catch can breather)..I am assuming I am getting pressure build up at higher rpms, but why? I also had my oil pan modified so it wouldn't hang below the k frame. The sump was cut shorter but it was widened so the level of the oil didn't change..could it be that the oil is not circulating past the Windage tray and building pressure?? Any help would be greatly appreciated...thanks