Re: Pup cavity
P-90s are wider (pole to pole) and skinnier than humbuckers. You will need to take off just a hair from the top and bottom of the route. You may also have to make the route deeper. First I would say don't do it. If that fails, get yourself a dremel tool and grind a bit like I said. You will have to make some holes to mount the ring as well. You should consider a Duncan or a real Gibby P-90 first I think. You can get really sweet distorted tones out of a good P-90 (people ALWAYS replace stock Epi humbuckers, right?). Another though is to look at an SG-X. It is basically an SG Special with only one pickup. I have one and I love it. It is my main guitar. SG-X are very cheap on Ebay ($300-$500). To buy another SG Jr., well...that will cost you considerably more (at least $500 for a Gibson). SG Jr. are very disireable, most people do not give an SG-X the time of day. I would say buy an SG-X or trade the Jr. you have now if you cannot afford another guitar. Most people regret doing what you are considering, and most guitarist learn the beauty of the P-90. Most later than sooner. Just my 2 cents.