The dual shock R65 bikes (and the somewhat short-lived R45 bikes) have a slightly smaller/shorter frame than the R80/R100 of the same time period. So, to maintain "stock" height and handling characteristics, the R65 dual shocks are shorter than the R80/R100 shocks by approximately 1 inch (actually somewhere between 1/2 and 3/4 inch) . I think that one can fit taller shocks, but it will change the ride height and handling in a not so great way.
If you order from Bob's or Capital Cycle.com or other airhead-savvy places, they usually will have the R45/65 specific shocks. The OEM ones are not well liked - I do prefer the Progressives on my LS. You can also fit Hagons, which can come in some range of lengths. Just be sure to tell the seller that you are looking for shocks for the R65 which are shorter than the R80/100 standard shocks.