Guys,
Do not use a rubber donut at the end of the gearbox in front of the propshaft if you are not running a support bearing on the propshaft and the diff and gearbox are not perfectly in alignement.
In my BMW engined Cobra I used the rubber donut at the end of the box, connected to a propshaft with UJ at both ends, all as recommended by the propshaft supplier, and what I got in reality was , to quote a fellow Cobra owner and far better engineer , " A bloody skipping rope" which would actually start vibrating like hell at anything over 20 mph.
I replaced the donut with a billet aircraft aluminium spacer and theres no vibration whatsoever.
When i'm fitting the prop to the gearbox in my next builds I'll be throwing the rubber donuts in the bin.
Cheers,
Tony