Hey, don't listen to Ed...it can tend to be a good deal harder than standing on your head while fishing wire through a L.P.cavity...lol. big thumb up for fluted reamer, and welding rod, stiff wire (like solid core household mains Gnd., coat hanger, or even old metal 'track' from windshield wiper-or wood ruler is good 'cause it's flat, and you get control when twisting.
Also might go with mock up of hole pattern, paying attention to cap placement/dress in relation to F hole, etc. especially if using big old style PIO's or HV caps with long leads. also,if at all possible use BEST WIRE...if you have to muck with it, you don,t want it fallin' apart/fraying before it's in.
Cover the old girl up with a cloth(leaving F hole open of course) taped down with low tack tape so as not to scratch, or stick if desired (and bench too), because you may well be turning, twisting like a couple Lambada dancers, the two of you before everything's back together. Funny, I used to do this all the time when I was younger, but was just doing my epi 'custom shop' 335 a fewMos back, and was a lot harder and time consuming than I remember. Fishjing the pots through with monofiliment, or? is a good idea.
I try to start at jack first, fishing through, tightening then moving on to next closest to jack, with parts staged 'ship in glass' sort of-all pre wired from my piece of Luann mock up. or cardboard, or whatever. all with fishing line pre 'fished' through pot/jack holes, and tied to pots/jacks, then fastened to body with tape so as not to fall/feed back through. maybe allow time to do in stages if it's not going smooth......seems it's one of those jobs that can go surprisingly easy, or turn into a headache, just as easily.
Oh, by the way, it almost sounds like you might be a candidate for a p90echtomy...they'll usually fit in with just a hair shaved off in the four corners of the cavity, & if you don't rout the width too much/or maybe even at all, can be covered back up with stock HB mounting rings if it turns out you didn't like 'em, but I can't see not liking them. I had Doug ''Raging Cajun'' Kershaw's ES 330 for a while, and Holy Cow, maybe the best 33(?)style guitar I've ever played/heard. traded my '52 LP for that guitar and 3mos rent. come to think of it, The LP had one of the best sounding p90s I've ever heard also. 330 was 60s if remember correctly. wine red, major checking, beautiful, in every way, like said...when Gibson, or Epi for that matter, got it right, man it was RRRighTTTT!
good luck, and hope I could help in somee small way. and don't be afraid of step drill...just watch out-some have more deeper cutting area than otherass---meaning the amount of cutting area before it 'steps' up to the next size hole on the same bit. ultimately you want cutting area same or more than thickness of body, or will start to cut next size hole. harbor fright has a set of 3 for less than one of some others. of course probably not as good, but my set has lasted a few years, and you'll use on chassis all the time. makes flawless holes in lot of metals, with added bonus---just kiss the top of hole in metal with slight pressure from drill when desired size is reached and you can run your finger around it, burr free!.