Maybe others have better ideas, but here is how I do them.
Radio sits in dash attached to the front console bracketing using the rear support stud on the radio, so yes, the radio will be already in place. The console is fully assembled except the shifter plate (same manual or auto trans), shifter should be out of park, in neutral or drive (auto) or directed rearwards on manual trans. Point the back of the console towards the headliner while the front of the console towards the gap or area ahead of the shifter. As you enter the nose of the console downwards and going up under the dash, swing the back of the console towards the floor some. Watch the shifter handle as it enters. You may need to adjust it some as you move or swing things together. ALSO, be sure and watch both dash front brackets as they get nearer to the console (all of this is happening simultaneously, hard but not impossible) Those brackets hanging up from under the dash, need to stay INSIDE of the console but also need to get "inside" of the compartment of the console.(the compartment behind the roll up door) The console should now be about an inch or two from being against the dash, pretty much in place as you feed the radio tuning shafts into the console face. These last two details are part of the difficulty, you are trying to get or keep those two forward brackets INSIDE of the console compartment (read as past the very back of the compartment) about the same time with feeding the tuner shafts into place. Just don't hurry it. The shifter is probably fighting you around the shifter opening too at this point, if the handle is off the shifter, it may be a bit easier (auto trans) but not required. Once the console is all the way against the dash and in place, it may be easier to locate screw holes with a fine steel "pick". start screws at the top of radio plate, probably the front two under the carpeting next and the back two last. The screws under the carpeting were punched in on the assembly line so with different or new carpeting, often difficult to locate them again. This same "pick" tool has worked for me in doing this each time. DO NOT OVERTIGHTEN THE SCREWS ON THE CONSOLE to the floor. (ask me how I know this
)
Radio nuts & washers can be tough to start without cross threading. I use a small pocket screw driver or the likes and if they will not spin freely, they may be cross threaded...be careful not to over tighten them.
The shifter plate goes on last after everyting else is secured. Protect the leading edge of the rear console top behind the shifter plate opening ( I would use tape). You will need to also protect the face edge of the lower corners near the compartment opening from scratching too (I would use tape there also). Notice on the under side of your console shifter plate, there should be a wavey spring plate on the back, staked onto the shifter plate. You are going to set the shifter plate into place and to get the shifter plate under the radio face plate of the console, you slide the shifter plate back over top of the rear console plate by lifting a little against that spring plate, so the shifter plate ends up over top of the rear console trim plate temporarily (hence the protective tape). Now simply slide the shifter plate under the radio face plate and remove your protective tape.
Key element here is to not scratch things up in the process. I've had mine in and out several times and also on various other 67-68 Mustangs or Cougars over the years.They were driver cars so I got a little "practice", at times learning by mistakes while doing various radio swaps, carpeting replacements, shifter light bulb replacement, dash repainting and so on...I'd say I've done this at least 20 times. (maybe more?)