Matt, 65mm, been there done that, got the t-shirt. Tried two different bodies, both ran like shit and doesn't give any BHP gains only VERY sharp throttle response horrible for around town driving. As for underdrives, I did them in conjunction with other mods so by themselves difficult to tell how beneficial they are however with more aggressive cams they will help in the lower gears for the engine to rev up faster as it is using less of its power to drive the ancillaries. The Kent Sport cams I have no knowledge of myself but from looking briefly at the specs they would give you an improvement.