When installing a 20 head in place of the 14, which head gasket should I use, the one for the 3.0l or the 3.5l? As I understand the 3.5l one would need a little trimming, but otherwise is somehow better. In what ways is it better & just what would I be trimming?

Ok, let me rephrase the question. Use a 603 head gasket, or a 606 HG which as I understand it would need a couple of holes drilled into it - but is metal vs composite(for the 603). When I asked the question, I remembered there another possibility out there but got the wrong engine as the possibility.

On another front, when the car runs, occasionally on shifting it "twitches" like a front driver exhibiting torque steer would. Rear end bushings?

Tossing a bone, here. Not a pro. I've got a #20 head on my 603 and used a 3.0 gasket kit $119? that I bought from Phil. There was no need to cut any holes. Head gasket worked perfectly, I know this because I rebuilt it again a few months later when the turbo blew up, and everything was beautiful. This was about 2.5 yrs ago. Be sure to follow the manual if you do this job. I'm pretty sure it was a Reinz gasket kit (blue packaging?), maybe an Elring. Dave-gsxr says Elring are slightly better (find his links on the "antenna garbage" post right below your post).

87 300TD 325k