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use a reduced pressure backflow

no

The RPZ (reduced pressure zone) device is the one choice that will meet any code requirement, no matter where you're located. As for elevation, it need only be a foot or more above ground, even if the ground slopes uphill from that location.

Just to add to what Wet Boots is saying...

An RPZ is very much like a double check assembly. Actually, it is a double check valve assembly, but in contains "extra" stuff such that a failure in the check valves results in water spewing out of the RPZ rather than having any chance of flowing back into the supply line. (The Reduced Pressure part comes into play because a part of what this "extra" stuff does is attemt to maintain a pressure loss within the device. If something goes wrong with the check valves such as a piece of trash get caught and make the valve stick open, the RPZ will not be able to maintain the pressure loss and this will cause the device to start venting water rather than allowing a backflow).

So just like a double check, an RPZ can be installed BELOW control valves and sprikler heads. However, if the RPZ were to become submerged or something block this "spew" valve, the backflow protection of the RPZ could become compromized, therefore the RPZ itself must be install ABOVE GROUND so that it's free to release water.

A double check assembly, on the other hand, doesn't have any way to release water (during NORMAL operation) and therefore a double check can be installed below ground (in a meter box so that it can still be easily be accessed for testing).

There isn't any rule that forces the PVB location to be at the point of the property's highest elevation. The only requirement is that the PVB device itself be at least a foot higher than any pipe that feeds from it. That means you could have it at the house, at seven-plus feet above ground. Hide it behind a chimney, maybe. Unbeautiful, yes. But a PVB will save you almost 10 psi compared an RPZ, not to mention the hundred-plus dollars difference between the two devices. A 3/4" device will be sufficient, although a one inch might be easier, since your sprinkler plumbing is usually one inch.

Using an extra 270' of pipe in the supply line will obviously cost extra money, but it will also result in a loss of pressure, because water moving through a pipe loses some pressure. The amount of pressure loss is a function of pipe size an water flow (gpm) (here's where to find a converter tool http://www.irrigationtutorials.com/formulas.htm#sec.

As an example, if you are using Sch40 pipe and designing for a flow of 10gpm, then the 270' of pipe will lose about 6psi in a 1" pipe, 2psi in a 1.25" pipe, and 1psi in a 1.5" pipe.

How ever, RPZ is going to lose about 10-15 psi. As for deciding what size of RPZ to use, you have to look at pressure loss tables and find out which size RPZ loses the least amount of pressure for your desired flow rate.

Two alternate ideas, but you'd have to find out what may or may not be allowed by building and plumbing codes.

#1. You say that the change in elevation in the yard is only 5-6 feet. Is there any reasonable way to install a pressure vacuum breaker about 7 feet in the air at the bottom of the hill? There might be various code that limit the height of the PVB, but regardless of codes, this would only be practical if you use strong metal pipe to and from the PVB, there is something (like a building) to attach the pipe to, and there is some reasonable way to access the PVB for testing without getting on a ladder. While it could be done, it might be so iffy (i.e. must be done just right) that codes may not allow it.

#2. Assuming your local codes allow a double check, you could improve its reliability (and the reliability of your entire irrigation system) by installing a WYE filter up stream of the backflow preventer. Last time I looked at the literature from companies such as Watts, they always show the installation of their backflow preventers being proceeded by a WYE filter. Of course this could only be suggested if you are not injecting anything into the irrigation water (i.e. fertigation).