The numbers are just a serial number, and mean very little. The YS was used several years on different engines.
Looks like the YS code was used on 350's, 389's and 400's, beginning in 1965, thru 1978..
http://www.wallaceracing.com/enginesearch4.htm
You'll have to get the block casting number, off the pad just below the rear of the passenger side head, and the 4 digit date code, from near the dist hole, to make a positive ID. :yes