The following applies to all Spittys. There are a number of things that can cause vibrations, but in my experience it’s rarely the motor. A well sorted Spitty is one of the smoothest sportscars of the era on the road.
Spitty tailshafts are notorious for being out of balance, this causes not only a vibration, but will loosen the pinion and gearbox flanges, which will cause vibrations just by themselves. If you have a vibration that comes and goes as you take your foot on and off the pedal at speed it will probably be a loose pinion in the diff.
Front and rear wheel bearings can and do cause vibrations, this is caused through a lack of lubrication.
Poor quality tyres are notorious for being out of round and/or the tread going around like a snake (these tyres are impossible to balance).
Exhaust systems on Spittys are always a problem. My suggestion is to adhere as closely as possible to the original mounting points and also check for correct clearance to the chassis.
Engine mounts although not normally a problem are worth a check. I have had more problems with rear engine mounts than front engine mounts, they seem to get very soft which allows the gearbox to move around a lot.
Distorted disc rotors are usually quite obvious.
I recently had an interesting vibration, which took some tracking down. It turned out to be an out of balance alternator, which I overcame by replacing the damaged alternator with an original type Lucas alternator.
Last but not least shockers are probably the most overlooked item on most cars; I cannot emphasise enough the importance of having good shockers. I personally run “Koni”.