Mechanical seals don't need to be "Lapped in" so no running up of the engine would fix a newly installed seal. The whole theory of the mechanical seal is that the seal faces are lapped to less than 2 light bands and there's nothing smaller than 2 light bands that will cross the faces of the seal.
Without knowing how you handled the seal components or the method of installation i.e. lubrication used and where, it's hard to help here. There are several areas of the mechanical seal that need to work properly to achieve a seal.
The stellite face has to seal to the housing so the leak could be across that sealing face. If the housing was not cleaned back to as new, that might be the problem.
The Stellite face when removed from the packaging as a rule of thumb, needs to be cleaned with either methyl hydrate or lacquer thinner and when ready for installation the face of the seal should be a mirror finish and I truely mean a mirror finish. You should not see any stains, on the face what so ever. When handling the stellite face, you should be wearing nitrile gloves. the acid in your skin can damage the face of the seal and what may have happened here, is that you installed the seal and didn't run up the engine and there was no flushing of the seal with the antifreeze and maybe, just maybe the acid from your fingers had a chance to etch the stellite over the past few months.
The carbon seal face needs to be inspected before installing for cracks or chips on the perimeter of the seal. There's also a seal surface between the carbon face and the seal drive shaft that the o ring rides on. Lots of reports of a damaged drive shaft at that o ring seal area. O-rings needs around 15% compression to be able to do their job so knowing that, and knowing that the seal drive shafts have needed replacement in the past, I would measure the shaft and ensure that 15% compression is there.
I always ensure that when installing o-rings that I use a silicone based o-ring lube or if that's not handy at the time, use the fluid of the system being sealed as a lubricant. In this case, I would use the antifreeze. Both the stellite seal face and the carbon face seals have to be rolled into position so as the seal is installed, the o-ring has to squirm into position instead of being in position and then compressed. The o ring has to distort as it's being compressed/positioned and if the o ring is dry when installed, it will not relax and orient itself so that the entire seal is sealing properly.
At this point, I hate to say it, but you really should disassemble the seal and inspect it and particularly the shaft in the area of the carbon face o-ring.
Let know how you make out. Here to help.