LMC (and other companies) sell new plastic molded kickpanels with 6 1/2" coax speakers in them. The LMC part number is 38-8510 and the price is $89.95. Kinda pricey and without knowing what brand of speaker it is, I would doubt it is a very good brand.
On the up side :
The factory kick panel areas are flat so you could make yer own using 1/8" or 1/4" masonite for a base then mount a ring to it to angle the speaker. Pick out yer own speakers, cover the new kickpanel in matching carpet, install the new speakers and mount the whole assembly. Easy stuff.
Two 4" speakers can be put in the factory speaker location in the middle of the dash. Using the same masonite, cut the appropriate size rectangle to cover the hole, cut the two holes for the speakers, paint black so it doesn't show, mount speakers, then mount the assembly under the dash. Easier than the kickpanels.
For a sub, w/o removing the tank behind the seat... how bad do you want it? That's what you have to ask yerself. Relocating the gas tank to the back would make the sub solution A LOT easier and provide more safety for you and your passengers if anything was to ever happen along the lines of a crash. There are several options for tank replacement and LMC has a kit for that too. Part number 32-5925 and the price is $199.95 There are cheaper options out there but a small price to pay for the safety you gain the work that will have to be put into the sub enclosure location. Think about it...
Pretty much the only way to accomplish this one would be to figure out what size sub you want and the enclosure size needed. A 10" or 12" sub should provide enough bass to get the inside of the cab rattling pretty good. Get under the truck and look for anything that is below the seat area of the floor. Figure out where you can move it to so you can cut the floor. Then get handy with cutoff wheel and cut a portion of the floor out for the sub enclosure to fit. Weld up a metal box to hold the sub enclosure and weld to the floor. Make the sub enclusure to fit new metal box and blah, blah, blah. If ya can't figure it out from there, you shouldn't be doing the job. lol
Amp placement : Next to yer new sub enclosure screwed to the floor under yer seat. OR If you got smart and moved the tank out back... you get the idea.
This next part is the most important point that I cannot stress enough! (If you want yer truck to sound halfway decent)
6 x 9's next to head is a HUGE no no!!! If you put them right next to head, all you will hear is the left rear speaker. Forget about hearing anything other than that and any bass you might add. Putting the speakers in the front kickpanels and the dash will provide a decent soundstage and keep the sound clear. You will be able to hear all your speakers and get a much better stereo sound. And don't count out the 4" dash speakers. They will bring the soundstage up and the tweeters will reflect off the windsheild for a nice balanced sound when mixed with the mids in the kickpanels.
Decide what speakers to purchase by
listening to them. Let YOUR EARS decide which ones are best. Don't listen to what yer freinds say, don't listen to what the salesman says, and try not to let your wallet sway yer opinion. YOU have to listen to them, not anyone else so make sure
YOU like how they sound. More often than not, what sounds good to you may not sound the same to anyone else so when it comes to speakers, don't let anyone else try to tell you what to buy.
OR
Run straight pipes and listen to the engine.
My 2 cents.
Regards, Dave