Stock mounts are hydraulic ones. There are two chambers and there's a flow control disc between the two chambers.
Here's one mount which I cut up when I had the same problem as you did have.
Pics under the "images\autot\volvo-misc\300srj\mount" -listing
http://www.elisanet.fi/mtn/#IDimages-au ... srj-mount-
340 structure is totally different from the 360 one as in 360 the whole engine hangs from the mounts and the engine mounts also supports the front weight of the whole drive train. Second mounts are at the gearbox end, so it's not anywhere near similiar as the setup is with 340 where the engine and the transaxle are separate units.
You can replace those 360 mounts with more simple mounts if you wan't but you have to make such mounts which are designed to withstand constant stress in application where something is hanging from the mount. This is the reason why you can't use normal sandwhich type rubber mount mount in place of a 360's hydraulic mount as if the sandwhich-type mount would fail, the whole engine would drop. Rubber mount has to be designed in a similiar way as the stock 360 hydraulic mount, so it is failsafe and will withstand the necessary forces. This means some kind of a casing for the rubber and also some kind of a support inside the rubber and casing which will hold the forces pulling the mount apart.
Easier way than making a failsafe rubber mount would be making new engine mount brackets in such way that engine is placed on top of the mounts, rather than hanging from them. Then you could use the common sandwhich type engine mounts. Here's a one example of this kind of conversion:
http://www.volvo300club.nl/discus/messages/29/12025.jpg
http://www.volvo300club.nl/discus/messages/29/12015.jpg
http://www.volvo300club.nl/discus/messages/29/12094.jpg
I chose it more experimental approach and drained the old mounts from fluids, put them on a vice and compressed those. Then I refilled the chambers with polyurethane adhesive and also the space between the metal casing and the rubber at the top. I used marine grade polyurethane adhesive (SikaFlex 291) and let the mounts cure for about a month.
I've driven with those now about ~50k km's and no problems what so ever. Engine sits at the height where it is supposed to be.
I don't see the solid mounts causing any problems other than vibration and noise as long as the mounts are made well and also the attatching points on the chassis and also at the engine side are up for the job. You might want to strengthen the mounts at the gearbox end also if you are going to put solid mounts in front as otherwise it might transfer the loads to the stock gearbox mounts too much.
-Marko
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