John,
In my relatively inexperienced opinion, from what I've read here (and on other online sites in in books) an MSD box would be overkill on your slightly modified, otherwise stock 250. The HEI alone should give you more spark than your engine needs.
Now if you had more extensive headwork (such as larger valves, professional port and polish, intake boss shaped/removed, etc.), higher compression and a lumpy cam then I'd say go for the MSD box.
-magic mike-
