I've done it, it's an easy swap, but do as suggested and if your donor is a 140, get the driveshaft, flanges and crossmember if possible.

Your existing crossmember can be modified - that's what I had to do. I had to cut a select area out of the member to clear the overdrive sump but was able to do it with a grinder and sawzall. I did not reinforce the crossmemeber after cutting and it has held up for close to 7 years of normal driving. If it was going to driven hard, I would reinforce it somehow.

I would also suggest using a 164 series engine mount for the transmission. They seem to hold up better than the regular B18 / B20 mount. Actually I use the 164 mounts for the B20 engine too.