Kevin, you need much higher rpm than the 67 is probably wanting to do if you use a narrow blade prop. The Brian Eather 12.5x5.3 3 blade that I use is slightly under cambered , and has a nice wider blade. My planes will not fly well at all on a narrower blade . When you say a 4 stroke run are you talking about a fast 4 stroke or a rich 4 stroke? If your lap times are too slow, try shorter lines, or more pitch on the prop. Mine use


s 62 feet lines 5.o lap times on a heavy 68 oz plane 630 sq inches. It has a fairly thin wing so I have to fly it faster in level laps but the maneuver speed is not fast at all.
My 67 did not like the flat back props at the rpm I fly at . I don't usually tac it but probably 8400-8700 ground rpm unloads in the air. I know you Spitfire has a big thick wing, which has a lot of drag. You may need to spin a bigger prop ( more diameter) I know my friend who flew a Windy U Spitfire used

Brian Eather 14x5 or  (2 blade)more pitch to haul his around . For sure use a wider blade prop at the lower RPM the 67 is turning. I had some 15" Eather 2 blade


For sure use a wide blade prop.
props I cut down  to 14" or less and they are fairly wide . I will look to see if I still have them . They are yours if you want free !!!!.

Props arrived Walter. Please check your PM. 😀
Kevin.