Here's a simple tool to align the Alfetta propshaft which can suffer from sagging engine mounts, transaxle mounts and centre bearing mounts.
The purpose of the aluminium spacers is to substitute for the width of a typical rubber coupling. Theoretically, if the shaft is straight, the matting faces of the flanges will be flat up against each other. If the alignment is out, there will be a minor gap created on the misaligned side. This can be tested and measured. Adjustment to engine location, transaxle position and centre bearing can be made to minimise or eliminate any gap.
The gap is measured between the spacer face and the inside of a coupling mounting flange.
Works well on the centre and rear flanges, the one behind the engine is a big ask.