I am planning on replacing my stock radio and speakers in my '85 K20. I will be putting 6x9's in boxes behind the seat, and I would like to know what size speakers will fit in the stock locations in the dash. I have been told that only 3 1/2" will fit, but that was someone trying to sell them.

Without modifications, the salesman is right, I think. But, with a big enough hammer, anything is possible.

I think a 4x6 or 4x10 will fit in the middle of the dash. Thats how my 85 looks at least.

3 1/2" will fit in the stock locations. I cut out the metal a little bit in mine to fit 5 1/2" in my dash. There is also a hole in the middle of the dash for another speaker that you can wire in their if you want, you could probably fit a 6x9 with a little modifications.
For your 6x9s, there are speaker boxes already made that will fit them just fine. I bought mine at best buy for like 40 bucks. they fit right in the stock locations, after pulling all the brackets and stuff out of the truck.