I--like most scale idots am trying to spin a scale prop with a high reving 2-stroke gasser. As you know this is futile. Large radial 4-stroke technology has catapulted "radial cowled" warbirds to a different level. The narrow cowled warbirds however are sitting on the outside looking in. For some reason I have never been a huge fan of the P-51; I do like a good challenge though and I would say that the Mustang or the ME-109 have got to be the most difficult to properly prop. I've seen guys put "inlines" in that narrow cowl but have never heard how the rear cylinder faired over time. I guess some mfg.s have used a slight offset to help alleviate the cooling issues. The only inline that I see out there currently for 1/4 scale class birds is the ZDZ 80 or 100 both being 2-strokes. I don't know much about them other than they are probably to heavy when combined with the drive.