I looked for some solid information before installing a custom stereo in my 240 wagon, and didn't find enough to make me happy. Here is what I ended up with, and maybe it will help someone else. Everything was easily available, installed without too much trouble, and works great.
Mine is a manual trans that had the factory radio in the lower position. After removing this, there was the lovely lower plastic bin ready for my sunglasses. I installed a single DIN head unit in the upper location with a readily available plastic face plate that fit the hole nicely and gave it a nice appearance. There was a snug fit at the rear of the head unit, but using my hand through the glove box bin, I was able to guide it in.
Front door speakers: most any 4" will fit without cutting metal. I used Rockford Fosgate Punch 4" with a 1/2" spacer ring and it fit perfectly. Still room for the manual window and the glass doesn't hit it. In the rear doors, I did not have any speakers at all. The door sheet metal allowed a 5 1/2" without problem after cutting the door panel with a jig saw. Under the passenger seat I used an enclosed, powered, kenwood subwoofer. The photo is with the seat all the way forward. In use, it is completely out of sight. All the wiring went under the carpet or under the plastic door sill panels. I'm really happy with the way it sounds and how smooth the install looks.

 

if it was a sedan, I'd have suggested cutting the back deck for 6.5" 2-way speakers, at least thats what we did with our 240 sedan. had lots of pookah. wagons have less options.