5 speed wouldn't make any difference. Do you mean 5 speed overdrive? I just did that to a 58 LCF that had a 4 speed.
It was very expensive and time consuming. Granted, getting all of the parts to make a small input shaft New Process 5sp OD match a Pontiac block/bellhousing is probably way more complicated than chevy-chevy. Bottom line...it only gained me about 200 RPM. 22.5's or 24.5 rear tires would probably net more

1947 M15 Studebaker 454 4x4
1949 Chevy COE
In the Stovebolt Gallery
More pix of the COE
1950 Harley FL
1958 GMC LCF 455 3x2bbl
1961 Chevy C60 Tilt Cab 327
1966 Chevy C60 Tilt Cab 496
1961 Chevy K10 sb 292
1974 Harley FXE
1979 Chevy K10 sb
2007 Harley Streetglide


I'm not that good but I'm slow