1988-1991 the calibration for the speedometer is done inside the cluster, there is no external DRAC. So the only way you could swap clusters and have the speedo still be correct by default is if the donor truck had the same rear gearing and original tire size as your truck - the odds are not in your favor.
It's not hard to calibrate those clusters, just a little more work than the 1992-1995 with their external DRAC.
I did a write-up on it, with pics, it's for a moonie cluster but the process is the same:
http://www.chevytalk.org/fusionbb/showtopic.php?tid/258562
On the tach it's because of what you already know: v6 vs. v8. Since the tach signal comes from the distributor, the reading will be wrong. From what I've read a few times, there may be something as simple as changing a resistor on the cluster but dammit if I can't find the original article that talked about it - it's kind of a rare topic since not many find the 1991 needle cluster and then need to fix the tach reading. :/
Richard