I have always had to loosen the other main caps, slightly, 1/32 (more in back and less up front). It allows the crank to slightly drop and take the crush off the upper half of the rope seal. You do not want to drop it too far up front, as it can damage the front seal.
Removing the upper main seal can be frustrating.
You may get lucky and tap the seal out. Use a brass or nylon drift (to prevent nicks on the journal) and gently try driving one end of the seal to run it up one side of the seat and out the other side.
If the seal fails to budge, carefully run a screw into one side of the seal. Use a long shank screwdriver and drive it by hand, to prevent slipping and to feel how the screw seats (I wrap the screwdriver shank with electrical tape to prevent the potential to mar the journal if I slip). Use needle-nose vice-grips on the screw and pull the screw & seal on one end, while you use the drift to push on the other end.
Do not pull the seal all the way out, keep it half-way out until you are ready to push in the new seal. The old seal maintains the clearance between the crank and seal's seat groove, and makes it easier to push in the new seal.
Prep the new seal with a light coating of Vasoline or other light lubricant. Once you have the new seal half-way around the journal, push it in while pulling the old seal out. Allow the new seal to follow the old seal out (it helps).
With the seal installed, use a single edge razor blade or Exacto-knife to trim the seal ends nearly flush with the cap & groove. Trim around the edges to prevent any seal "hairs" from getting caught under the cap as it is tightened. Don't worry too much about a rough texture with longer "hairs" in the center of the seal ends, these will crush together and help the ends of the seal to mate and mesh. In the old days the seal was ready to go with a dab of grease on the ends, and many builders these days add a drop of silicone on the seal ends (I do not, and never had a problem).
Loosely fit the cap, and check for alignment. Go back to the other main journals and tighten them first, and try to run each of the other main caps up into the block with the same torque and clearance to evenly seat the crankshaft. When all the other main caps are snug, then go back to the rear main and run it up snug.
Test your torque wrench a few times against another wrench to get it working (what is needed for most "click" type torque wrenches). If the wrench is good, torque the caps down in steps (I always use three steps of torque). Watch the torque ratings for the caps and bolts (they are not always the same).
I was also taught to pull the front main cap for inspection, because it is the last to receive oil (and the first to show wear). If you have the crank lose, you might as well check it out.
This was how my father showed me to pull the rope seal on my Pontiac's, and I have used it many, many, times (including the 327 in my 63').
PS, in the proof read I caught DZAuto's advice to rotate or stagger the ends of the two seals, to locate the mating ends slightly off of the crank & cap center-line (damn good advice, if you can slip the seals).