I hope I responded in time before you left.
My buddies and I have our annual trip on the John Day , we have done it now 4 years. We always float Spray to Service creek , its about a 7 or 8 mile float. I say float loosely though be prepared for a bit of rowing as the afternoon wind blows right up the canyon up river and in the froggyer sections you have to work to get down river. This float is sort of an all day trip because of the lack of moving water and all the rowing that must be done. There are a couple other places on your way toward spray you can launch your pontoon boats that are a bit shorter, Muleshoe is the only one coming to name at the moment. The fishing is pretty amazing , between the 4 of us we typically catch and release 250 - 300 fish in a day and miss that many more due to slow or poor hook sets. Later in the year like now the fishing is really fast paced every cast you will be landing or missing a fish. So many small ones that you will be lucky to hook one over 2 or 3 lbs.
I would give you more advice on where or how to fish but this time of year the small mouth are literally laying everywhere. and by everywhere I mean everywhere you will pull small ones out of the little current or trout looking water. The bigger Smallies will be laying in the slack waters where the river bends and creates a dead deep pool. Topwater baites have not produced well for us , crankbaits , buzz baits and texas rigged senko , robo or any kind of plastic.
I can promise you with almost certanity you will not be disappointed. I wish I could give you some advice on what its like fishing below service creek , but we have never been down below there. Off hand I can not think of what I called some of my other John Day posts just search me and you will find them.
Best of luck to ya bud, we are heading over there on the 18th , save some fish for us please.
Jer