I have an '88 also, just replaced the sheaves a couple months ago. These are what I used, just the two of them @ $8.95 each. Took only a few minutes to do.
http://www.catalinadirect.com/index.cfm/product/124_623/masthead-sheave-internal-halyard-c-22.cfm
http://www.catalinadirect.com/index.cfm/product/124_623/masthead-sheave-internal-halyard-c-22.cfm