As someone has already pointed out, the propshaft has a sliding yoke, the axle should never be able to compress the prop enough to push on the gearbox engine assembly.

The thing that stops a PV engine and box from moving forward is the gearbox mount. The engine mounts on a PV are not angled, unlike those on 120's and 140's.

On a PV the gearbox mount is therefore a cast loop on the back of the gearbox with a metalastic bush and a bolt running right through it. I've seen PV's, especially 444's and early 544's with B18/20 engines retro fitted where people have used a 120/140 series style rubber cushion mount for the box, nothing much other than the bond twixt rubber and metal on the mounts then stops the whole lot from moving forward.

It's where I'd start looking.

Regards


Pete