So, I plugged the vacuum gauge into manifold vacuum after installing the new Motorcraft 2100 (haven't driven it yet, just running it in the garage). I had pretty low vacuum before, with the Holley carb on it but could not identify a vacuum lack around the carb anywhere. Now, the vacuum gauge reads much better, though still a tad low. That might be a timing issue which I have not yet addressed. I used to think that the timing was way off with the low vacuum and a typically hot running engine. Now I think it's off a little but not horribly. Still have to check/adjust it. Anyway, the gauge is now very jittery, which it was not, before, with the Holley. A jittery vacuum gauge supposedly indicates worn valve guides. This odd fire 231 has less than 10k since rebuild. So, first, why would this symptom suddenly appear? Was the previous low vacuum hiding the jittery gauge? Second, it was my understanding that these engine do not have replaceable valve guides. Not sure I'm right about that but, that's what I remember. At under 10k, it seems odd that thy would get worn already. Finally, is it really even that much of issue to be concerned about? I'm not interested in pulling the heads off this thing and having them gone over again, with knurling or whatever the repair might be. It's not a daily driver and honestly, sees under 1000 miles a year, though I'd like that to increase but, it probably won't. Next move for me is check the timing and get it right but, I'm curious what you engine guys think about the situation as it stands today, with the jittery gauge and it's potential affects, long and short term. Thanks folks.