Piece of cake. Chocolate, gooey, and very do-able for a DIY-er. With care.
I, too, lost motor mounts after a rear-ender. The other guy paid. My experience is when I did the job on my 1979 245DL.

First read all the books you can Haynes, Bentley, FAQs on the BBRD, Volvo "Green", etc.

Overview: The engine must be lifted to get weight off the mounts. Each mount has three sections: The bracket on the engine, the rubber mount, and the bracket on the crossbar support.

Here's what I did. Car up on front ramps, E-brake on. If an autotrans, in Park, if a standard, in Neutral. Chock the back wheels, too.

Cover work area with many layers of old newspapers. Then drop the gravel shield. Use a large flat tip, or your preference, and clean off all the road greasy grimy stuff around the brackets on the crossbar.

Remove the bolts that hold the Left and Right brackets to the crossbar.

Now you can lift the engine. Doesn't take a lot of distance. I used a bottle jack pushing up onto a 2x4 block across the oil pan, clear of the drain plug. Watch the back of the enigne, it can hit the firewall.

I was able to control the engine lift while underneath, a handy arrangement.

Do the RH side first. Loosen all the nuts on the mount and remove the bolts holding the bracket to the block. Then you can lift the entire assembly out. Put the new mount in loosely, and gradually lower the engine. When all the bolts line up OK, tighten all but those holding to the crossbar. Do those when the weight is on the car and the other side has been done.

The LH side I had to do a little differently, you will figure it out as you go. Same type approach. Could be that I had to attach the RH mount and then lift the engine, causing the block to pivot and getting the necessary clearance. Also could be that I could not get the 3-piece assembly out, so I had to take it apart in the car.

Be very careful, having all that weight overhead is potential injury.

It's a dirty but easy job. Allow a whole day, including cleanup. (Vinyl gloves are great) Also get ready to replace the transmission mount. It no doubt suffered damage in the accident, too. A weak tranny mount causes the front mounts to weaken and fail prematurely.

Good Luck,

Bob

:>)