Need some advice

Your trannie has a very tall 1-2 step and the rear is low, so the engine has to have a very wide RPM range in terms of percentage RPM range (not max-min RPM's, but max/min RPM's). You won't have that wide RPM with this long cam duration.

You'll always be suboptimum with the 3 speed manual so another option is a 4 speed to reduce the % gear change steps in the gears and does not demand as wide a % RPM range. You might end up pretty good with that, a shorter cam duration for sure, and (IMO) a gear in the 3.55 range to keep the RPM's down at 75 MPH on the highway. For me personally, the 3.91 would be too high revving for the highway. (And, of course, an auto trans has the feature of not needing as wide a % RPM range with even just a mildly higher stall TC.)

Do you know your advance curve numbers? Is this car/engine mainly for the street?