Dennis,
First, make sure you aren't doing anything that would tend to cause the crankcase to run under pressure. How is it vented? Next, getting all the gasketing detail just right at the rear of the sump is not easy. Third, setting up the "oil screw" details at the back of the engine is critical. Fourth, are you sure it isn't overfilled? Fifth, there is an oil seal conversion kit available which installs an actual lip type positive oil seal at the rear of the crankshaft. Now, don't you wish you knew all that BEFORE you put the motor back in?...
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon