A quote from the 1948-1951 Chevrolet truck shop manual in bold type:
“Note: It is important that the centering gauge be used to align the timing gear cover so that the harmonic balancer installation will not damage the seal and to provide uniform seal tension on the hub of the balancer.”
I read this as the seal needs to be held in a position concentric to the crankshaft when the bolts are tightened to function as designed. An old balancer with the bore polished out a bit to slip on makes a perfect cover alignment tool. It’s #17 on this page and IMO skipping this step is asking for a leak:
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0646.htm
