I have no experience with the Bennett exhaust, but itīs said that it offers very little resistance, so personally I would go for a 26x8 for your Pup.
As previously mentioned, I use almost exclusively Fiala wood props, as they load a lot less than other brands, and you are therefore able to use a larger prop than with other brands.
If using a higher loading prop, I would go for a 24x8.
With the stock exhaust on the G-62, the later model with the two outlet pipes, I got about 6100 rpm with a Fiala 26x8, and it powered my 1:3 scale Tripacer very nicely. Actually I rarely used more than 50% throttle, even for takeoff, since using full power had the 37 lb model leap off the ground in less than 30 ft.