Overton Mt is BLM while the Flying V Ranch land is private.
The Halleck Canyon volcanic rock is on BLM Land and has a county gravel road going through the canyon offering some roadside walls. There are some moderate to steep shorter walls here often having small crimps for holds.
You can visit Wyoming's Oldest Tree at the Flying V Ranch (please register at the main ranch office) and from here you can see the rock wall of Post Canyon and Tower Canyon. While here you can request permission to climb on that rock wall west <1.0 mile away.
In the vicinity of MM 35 on Highway 34 in Sybille Canyon turn west on County RD730 and head west 5 miles to the BLM Lands of Overton Mtn or continue west to Flying V Ranch.
Local Information for Halleck Canyon and Flying V Ranch Rock