I had the same problem on my rebuilt engine, and posted it here on this BBS, and was reminded of something that I saw whilst surfing all things MG but chose to ignore. Hey, if I didn't do everything twice, what would I do with my spare time?
Anyway, I redid the head as per below (from University Motors, I think), and have run it for 15 minutes or so, so far so good!
MGB WEEPING CYLINDER HEAD: The rule is, all rebuilt B series engines weep between the block and head, between the 2nd and 3rd spark plug. Some actually piss. To remedy this problem: That center exposed head stud, RH side, between #2 and #3 is the guide stud -- the hole in the head is 3/8 whereas the other holes are 7/16. Clean out the stud hole in the head with a 3/8 drill. Polish that stud, at least. Ensure a chamfer at the threaded hole in the block. Chase the threads on the stud, nut, and block (3/8-18 and 3/8-24). Ensure the head is planed to 0.001." Ensure that the top of the block is cleaned to a SMOOTH finish. Fit the studs back into the block with only about 5 lb-ft torque. Place a THIN film of clear silicone, RTV, sealant on the RH side of the head gasket. Into that EXTREMELY THIN film, place two strands of stranded, flexible wire about six inches long, twisted together, along the outboard side of the gasket, between its edge and the water jacket holes so that the thickness of the head gasket is effectively increased by several thousandths. Goodbye leaks.