I listened -- read your post TWICE -- your facts just don't hold much water. You are recommending a gear ratio that doesn't exist for his application... so your facts are based on erroneous data.
As for driveline critical speed... a typical 3.5" mild steel driveline of 50" length has a critical speed exceeding 9000 rpms...
So if you crank in your hypothetical 2:56 gear set - and a 1:1 final drive - using a 26" diameter tire... He'd have to exceed 300 MPH. Pretty impressive - but hardly worth discussing.
Using an overdrive trans - as above with a 3:73 -- he'd only have to exceed 150 MPH to achieve the magic critical speed number... again - I'm thinking he's not going to be running the car that much at these speeds. If he is - he can order a driveline that will handle it with ease.
