Oil leaking where tailpipe meets headders??

[eastcoastcivil]

I recently bought a 67 swb fleetside C10 with a rebuilt 350 4 bolt, edelbrock intake and a 600 performer carb. It sat for a year without being started at the PO and when I got it home it wouldn't start. The gas tank was really rusted bad inside so I put a new tank, sending unit, lines, fuel pump, fuel filter and cleaned the carb. I had it started for a little bit with the choke on and as it was warming up it started to run really hard and sluggish. a day later I started it again and it ran fine for 5 minutes and then sluggish again. When I gave it a shot of gas or turned it to half choke it would stall. The times when it would stay running when I gave it gas it wasn't accelerating like normal, it was really really sluggish. I then looked under the truck and noticed that oil was leaking from the connection at the headders and the tailpipe (right before the mufflers, and I mean a fair amount of oil). I checked the oil and it was a little bit too full but clean. Has anybody ever had this happen or know what might be the cause of this? Is my motor toast? Really really kinda down about this since I thought I was going to solve the problems with a clean fuel system and have it on the road this week?