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I believe they are 6.5" round for the front door speakers. You can go to places online, like Crutchfield.com and enter your veh. and it will display the factory stereo info.
6" speakers should fit in the doors without having to enlarge the holes. I had an 83 F-100, that had 6" speakers in the stock holes, and I added 6x9 speakers into the door panels just under the armrest.
Most 6.5's work in the doors, but there are some that are large 6.5's and need a little work to fit. Actually I think there are actually 6.5 and 6.75 speakers. Maybe it's the 6.75's that need a little fitting....
Also if you look at my 86 in the gallery (https://www.ford-trucks.com/user_gal...=50720&width=0) you'll see the sub box I built. $80 2x10" pep-boys sub kit with 120w amp, $25 in wood (3/4 mdf 4x8 sheet) and $10 in black carpet. It sounds great, but I would use 8's if I did it again. The enclosures are only about .4 cf.