As panzerkw said -- go directly to the NYS DMV website. Also, request the rules and regulations for the parkways. Caution, a cap alone does not do it! There are other modifications required... and, depending on the Trooper, you may be asked to verify them. My '99 4X4 QC Dually had a cap only, and was registered as a passenger vehicle. The troopers were kind enough to point out that:
1- Passenger plates or not, no vehicles can be over 20' long on the parkways.
2- DRW vehicles are not allowed -period!
3- Caps must be a minimum height (45"+?) above the bed
4- There must be permanently installed seats with seatbelts
OR
The vehicle must be converted to a camping vehicle with a stove and bed.
After being stopped 3X with no tickets, and 3 more with tickets, each one escalating in cost, I gave up driving on the NYS parkways.