If the frame is ok, and there isn't to much rust then I would pay 2500 for it. If the body is pretty straight maybe more. Things to look out for in old Mustangs are rusted over floor panels, rusted out seat humps (where the seat goes), rusted out trunk panels, dinged and bent up quarters and fenders, front disc vs 4 drum brakes, cracked or repaired shock towers, front and rear suspension quality, and what kind of condition the engine is in. I should also mention, northern Mustangs tend to suffer more rust damage than ours down in the south. Salt is a big no no for these cars. Just an FYI, I was offered 6000 for my running 68.
There was no V6 64-69 Mustang. What is there is probably a 200ci i6 1v which is a work horse if taken care of properly. If there is in fact a V6 then I wouldn't buy it.