I'm not positive on the allowable runout and that could be your problem. I'll tell you what I do fiest then what I've next.
What I do. I pack so much of the seal in that almost none is cut off. I leave a slight amount extra out of the block side, peaked slightly to push into the cap side. I have already drilled and taped a 6/32 hole about 1/2" up from the split and prior to putting in the crank I screw in a pointed allen set screw into the rope seal to hold it from spinning. I then put on the cap in which the rope is also packed as tight as possible. It is cut off flush and the point of the block seal enters the cap side and torque it to your setting. I have coated everything with assembly lube generously.
At this time it takes at least a 3 foot bar to turn it over...IT IS TIGHT. By the time the rest of the engine is assembled it does get a little easier. A 12 volt HP starter has no problem.
This assembly method held at 5500 rpm with 70% nitro and did not leak. Not a dry sump under vacuum either. It had positive pressure in the engine.
Your engine may need some help with vacuum and if this is a street engine I suggest a PCV valve. This will help keep a vacuum on the engine.
I don't do this but I've heard it works. A Dodge 360 real main seal is slightly larger than a stock GMC. I have heard to cut approximatly 1/8" off of each end and place material behind it (I've heard a ty-rap is the right thickness) and put it together as a normal split rubber main seal. I have one of these but yet to try it.
I have also heard a 225 Mopar Slant 6 seal will work. I have purchase one of these too but yet to play with it.
Others may have more to add.
I'm going over to the Fergusons this week to measure one of there record breaking motors. I believe they use the 360 seal and I'll ask them about it....Good Luck
