I've always just used the stuff for that purpose from the auto parts store. But yes, razor blade and iso alcohol first, get it all CLEAN! First thing to do though is mark the outside of the windshield where the mounting block needs to go. If there's any sign left of where it was, old adhesive, etc., just put a mark with a Sharpie or piece of masking tape over that on the outside. Makes it a lot easier to locate the mounting block.
Don't try to mount the whole mirror, just do the block and put the mirror on it when it's cured. Might put some tape over the block to hold it in place while the adhesive cures.
And don't put the block on upside down or sideways - that'll make your ass red.....