To keep it an inexpensive upgrade, you might also consider getting a set of friction bar-end shifters which would work fine with 6 speed (or any other freewheel) if you are OK with giving up indexing.

Good thread on this at;
http://www.bikeforums.net/showthread...r-End-Shifters

You could also possibly make your own 6-speed index bar ends using rivendale shifter pods combine with your existing shifters;
http://www.rivbike.com/product-p/sh13.htm

Might also be possible to move existing 6-speed shifters to the downtube if you can lengthen or fabricate a larger diameter clamp for the existing shifter base?

The Centurian DLX looks to be a fairly decent butted CrMo frame (Tange Infinity tubing). It came with some fairly heavy & junky compontents but I think it would be worththile end result if you upgrade the components, the frame itself is worthy of the upgrade. (Unlike trying to put dura-ace onto a huffy). If cyclocross type riding is your interest, you might also consider having cantilever brake bosses brazed-on in order to also facilitate the upgrade to 700c rims.