Keith,
There is a custom tank which I purchased for my R54. I too use the PST600R. I beleive with this 70 ounce tank and BVM UAT I am getting about 8 - 8.5 minutes of flight time. Of course I am not flying at full throttle the entire time. I usually set my timer for 7:00 minutes and try to land with enough fuel for at least one more go around if I have to.
If you are new to the PST 600's, you have chosen an excellent engine for the R-54, it is a great combination. Make sure to not leave your fuel shut off valve open after landing, or you WILL get a wet start, don't ask how I know this.... the tank sits up a little higher and the fuel will drain into the combustion chamber. Other than that, now worries. You can find the custom fiberglass tank in the Reaction 54 thread. Cannot remeber the exact price, but it fits really well into the fuse. Good luck
Tommy