It is pretty simple to support the engine with a small scissor jack and a small piece of
plywood under the jack and between the jack and the
oil pan.
You can then unbolt the brackets one at a time that sit on the mounts and swap out the mounts one at a time.
You should be able to keep the "settings" ie. turns on the threaded studs that go into the brackets fairly close and once all the mounts have been replaced, then call the $200 per hour
mechanic and just have him do the alignment.
If you are not "all thumbs", read up about aligning the flanges and get someone to help you adjust the mount heights and positions while you measure with the feeler gauge.
Be aware that
Yanmar mounts for 3GM30s are of two different hardnesses. If you need a shop manual that describes all this, I have a digital copy I can
email or I think the
Yanmar shop manual is posted on this website, somewhere.
I found Bay Shore to be quite reasonable with their
parts pricing, but changing engine mounts and aligning the flanges should take about 4 hours max.