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.