Impedance is a rather nominal notion, and 'exact' figures are fairly meaningless, really. The actual impedance is only for a specific frequency and power input. There is no noticeable difference between 8 and 8.3; treat is as 8 and you'll be fine. Would it be suitable for guitar..? The answer, I'm afraid, is 'a lemon; suck it and see'. If it sounds OK, it's OK. Be aware, however, that typical PA stuff, 'back in the day', was not rated highly for power handling, as only vocals from a modest amp were expected. Don't connect a 100W Marshall amp turned up to 11, as it won't last long, I'd say. Vintage gear can be nice, if played in vintage manner. Watkins Westminster amps from that period are now collector pieces, but their 17 watts won't fill a stadium..! Go easy on your find and enjoy it for what it does, not what you think it should do.