Stock compression ratio is 9.2:1 which is not enough to gain any mileage or power gains on CNG or propane. I wouldn't do it unless propane is REALLY cheap in your area.

To gain from CNG or propane, you can shave the heads as far as possible to bump C.R. While doing that, get hardened valve seats installed in your head. This will probably cost $1000, but check into rebate programs in your area.

Easiest & Cheapest: E85.