Thanks for all the replies. I agree that the 3.4 is the first choice. And if one is available before swap day, which is very soon, then 3.4 it is. 3.1 is preferable to the 2.8 but I have had a lot of good service from S10 2.8s.
The point on old American iron being cheaper to work on is also a factor in this swap. The old 2.6 has performed well but now needs headwork and I am reluctant to just rebuild the head with 180,000 on the motor. Thanks again.

Phil