why not raise the gearbox mount by say 5mm on spacers ( obviously being aware of the bellhousing touching the chassis etc ) & as you say the engine mounts by 10mm, I would imagine the gearbox mount could cope with a 5mm variance ( happy to be told I,m wrong tho ).
The prop has U.J,s so they would cope with the very little misalignment I would've thought too.....tho I,m sure I've read somewhere that prop shaft U.J,s should be slightly misaligned from gearbox to diff flange to stop them doing something that isn't good for their service life expectancy.( can,t think of the technical term
)
cheers
andy