why so slow??!!

I have a Stitts SA-3A playboy I picked up recently. It has an A-75
Continental and a 67" Fottorp prop with a 47 pitch. The plans indicate this
machine should fly around 120 mph with this engine; no mention of prop. A
previous owner I spoke to said it cruised around 118mph for him. For me, it
cruises around 95mph; I verified this with GPS. On run-up, I get about 2250
max RPM. On take-off it easily red-lines, suggesting to me the prop is not
a good match, but then I know next to nothing about this. It flies
beautifully, feels very well rigged. I won't pretend I know what the
problem could be; but I do know it is flying a lot slower than it should.
Any suggestions would be greatly appreciated!!
Thank you
Heino