I own two of the side exhaust 55s and the 61 so I can't do a direct comparison. About the only thing I know about the rear exhaust version is that it has a different power curve than the side exhaust. Some say the rear exhaust version is more powerful but I can't comment on that without real world experience. I would rate the side exhaust 55 as one of the best engines I've ever owned and that includes quite a few engines over the years.

The 61 is a very good engine but overall I prefer the side exhaust 55 and that would be my choice for the P-51. The rear exhaust 55 would be easier to mount and the muffler location is convenient but I believe the side exhaust engine is a bit easier to cool.