Sorry for novel, this should help!
Ok, I've done about 8 blow through includeding my own.
You can only make the hole as big as you BED wall will allow,
Your CAB wall is bigger, but you want the holes to be exactly the same size, within about 1/8 difference as possible.
Basically you get in your bed, and make a decision on how big your hole is going to be.
I have a step side, so I didn't make the box wider than my wheel wells, so I could slide the box in and out as i needed, without remove my tono cover, or needing someones help!
So I just followed the bed floor ribs up the the bed wall and drew a verticle line from the floor, to the little lip at the top of the bed wall.
It's up to you wheather or not to leave a lip at the bottom of the hole.
If you leave a lip, it should be at least 3/4 inch high to attach the crimp seal on the accordian rubber seal, similar to a door jam seal crimp.
then I usually leave a 3/4 inch lip at the top of the bed wall down from the angle on the top bed rail, you'll see what I mean when you look at the rail.
I tend to cut the hole level with the bed floor, so the box can slide in flush with the cab.
If you leave a lip, and want to slide the box into the cab, you'll have to shim the bed floor with wood, or metal, effectivly raising the bed floor so the box will slide in the cab.
Many people make the box bigger than the hole and push the box up against the bed wall, it is harder to seal the subs from the bed to the cab this way, but you get a bigger box.
When I cut the bed wall level to the bed floor, I use a metal flange,
duct sheet metal, galvanized and make a flange, rivet to bed and rivet to cab then use urathane caulking to seal.
then the acordian seal only has to go up each side and across the top.
and no, the twisting of the cab and bed did not mess this up, as I had this in my truck for 4 years, and no problems.
so, I used an angle grinder with cut off wheel to cut the bed wall out.
You SHOULD round the corners, not 90 degrees, the crimp seal on the acordian rubber seal doesn't seal as good in 90 than a nice rounded corner.
I used the bottom of Tim horton's coffee cups to make the curve.
I then use a two inch hole saw to make the curve cuts.
So after you have the hole cut in the bed wall, THAT is your tamplate for the Cab wall.
I then use the hole saw to make the corner cuts in the cab wall from inside the bed. Make the 4 corner cuts, useing the bed cut out as a guide and they should line up real good.
I then use a sawsall with a LONG fine metal blade and follow the line from the bed wall cut from corner to corner on the cab wall from inside the bed.
there are two layers of metal in the cab, you'll see this when you remove your inner trim and carpet from the back wall.
So you have to use a LONG blade
Use a fine blade with the sawsall, if you use too coarse of blade, it may catch the thin metal of the cab wall and warp it.
So, now you have a blow through.
Clean up all the edges with a grinder, or whatever.
paint the edges, and let dry.
If you use silcone, clear or black with the crimp seal, there is a little movement between the cab wall and bed wall, I find that silicone breaks down quicker,
I always use windshield urathane, Than stuffs holds good!
So you'll want to cut your acordian seal and tes fit first before applying urathane of coarse.
If you left a lip on the bottom, you should put the joint in the middle, so it has nowhere to leak, then just urathane the joint closed.
You'll want some wood and sticks to "prop" the seal when you get it in place.
I always mush the urathane into the crimp seal, then put it on the metal, now, it's called Crimp seal, cause you can Crimp it to hold better, but the urathane will grip good enough for you.
Once you have it all on, box line the whole seal with wood and jam the sticks to press on the seal while it dries, in the corners too, this will make a good seal.
Clean up your edges of urathane, if you used the right amount, you shouldn't have to touch it!
To run wires to the subs.
If you bring your box flush to the inner cab wall to make it all pretty, you can get the wires into the box easily.
If you have the box in the bed, you have to run you wires under the truck or:
I use black pool hose, or Shop Vac hose.
I cut the hole with a hole saw, beside the blowthrough hole as close the the inner bed side as i can and as close to the floor as I can.
I think the hose is 1 3/4 hole, but not certain.
Then I cut straight through the cab wall.
Then I push the hose through both holes, urathance for a seal and let dry.
then I trim away the hose from inside the cab flush to the urathane seal.
TIP: If you leave your box in your bed, and the box is bigger than your hole, you should make a flange, or port to bring the sound dirrectly into your cab, this way you won't lose any bass into the bed.
I used a ratchet strap on the two tie down hooks at the front of the bed to hold the box tightly against the bed wall, worked great.!
I made two boxes for my truck, one for everyday listening and one for competing.
My every box had two 10 inch PG XS which sounded real good.
and my competition box had one 15 stroker.
If you make your box to come in flush with your cab wall, you don't have to recess you subs into your box, make them flush with the wall and make a port, sounds good!
Hope this helps!