I was looking at the specs for the Fender Super Bassman (not the Pro; the blonde/oxblood one here) as well as the matching 410 and 610 combos (found here and here respectively). I've noticed the wattage of the head is well under the rating for the cabinets. Should I worry about this? Will it be an issue? My guess is Fender did this simply to make the cabinets more compatible with other manufacturers' heads.
not going to be an issue at all. The Super Bassman is a 300w tube amp and tube amps can put out peak wattage well over their stated power rating, as much as 2x (as is the case with the 300w SVT for example - which is known to be able to kick out as much as 600w peaks). And there is never really an issue with running an amp that delivers less power than the cabs are rated for, only the opposite - an amp that puts out too much power for the cab and ends up damaging the drivers.
That's not an issue either, if you hear the speakers distorting from being pushed too hard you just turn it down. Most speakers will start to distort heavily around half their power ratings.