Hi, I just finished a dash replacement project in my 142. Volvo from Heck has your instrucment cluster removal down pat.
Under the steering wheel brace there are two 5/16 inch bolts with domed head. You will have to take a dremmal or fordham hand tool and cut a slot in the top of the domed bolt head to allow removal with flat head screw driver. Not really a hard thing to do at all.
Technically it is not a hard job to do at all. You will not need a manual. Just very tedious trying to fit it back together. One trick I remember using is to line up the bolt and screw holes with a small phillips head screwdriver This will help position the dash so the other screw holes line up.
Look at my earlier posts for pics. I would recommend replacing the radio at that time as the dash will need cutting with your dremmel for the new music box. Also, I got some excellent results glueing mahagony veneer on the vertical horizontal dash plate that the gadges attach to. Just glue it on, cut out the holes with a razor. The paint with clear coat whatever. The origional Volo fake vinyl wood dash was a really dark dismal color. I used a nice light mahagoney veneer easily obtained from local lumbar store.
Now would be the time to replace the heater fan or at least degum the bearings. If you go into the heater box, be sure to save the clips that hold the two halves together. They can get away from you in those tight quarters.
Reassembly of the heater box is problematic in that all the components of the lower unit must fit together before the clamps will work. That took me the longest.
When reassembling the switches and gauges to the dash, make sure the orientation of all the wiring attached to above is not shorting out on other components behind the dash.
Good Luck
Joel
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Antique Swedish Steel 71 142E color V#102