The minimum length specfied in RFC 2131 is 312 and some DHCP servers take advantage of that minimum. (Such as Ascend pipelines). Pump is currently set to 340.
There is a patch that fixes this and other problems in pump (as well as reporting errors in syslog) at http://www.cse.ucsc.edu/~breed/pump.patch.
I've fixed this in CVS, and the fix will appear soon. I'd appreciate it if you'd like through my change and see if your result and error mods are still appropriate; I had too much in the way to be able to just apply them. If you're interested, I'll be happy to mail you a snapshot of the source code.
Fixed in pump 0.7.0.