My new dashpad just arrived from High Performance Auto. Eric suggested using the black 3M "gorilla snot" for the installation on my '64 220. I'm looking for all the suggestions I can get, since I have never done this before.

I used construction adhesive. I brushed it on both the metal dash and the underside of the pad. I did do the work on a warm day. Holding up well more than 20 years now.

I can tell you what didn't work so well for me.
I think it had more to do with how I held down the pad while drying.
I used 3M777 spray, which I have used for all sorts of applications successfully.
In the end the front has pulled slightly away from the metal leaving a 1/4 inch gap. I also damaged the thin chrome strip at the bottom of the dash which didn't help. So be very careful removing. As I recall it has some small nuts on the back side at the ends. Holding the pad in place while drying is very important. I think that's where I messed up. I wish I could give the right advice on that. Maybe someone else can chime in on a good way to hold the pad in place.
Good luck!
Gary

it also helps if it is warm, and use lots of the blue masking tape. 3M, not the cheap stuff.

When I did mine I just used a PVA glue called Copydex (I'm in the UK) This is a lightweight glue used for craft work, paper etc. IMO you don't need much at all and when you take off an original it just seems to have been randomly glued in a few places. Likely all down to how the fitter was feeling that day. It's held in all across the top bye the screws through the screen trim. The lower edge held by the dash trim and bolts so there's not much left to hold down. Fit around the speedo can be poor and you can only tweak it a bit. The very ends can have a dab of glue on them to neaten them up. Less glue over the main area means that it can move a bit when expanded by the sun. I don't know what material these new dashes are made with. Original early ones rarely cracked but a change of material about '65 meant that the ones after invariably do.

Agreed...My '62 Amazon, 65K miles, original paint and interior, has it's original, uncracked dashpad, which looks as good as it did about 35 years ago when I bought the car...

Greetings from northern NH,

M