Since my 68 Dart is radio delete, I just bought a small plastic enclosure on ebay for arounf $13. I have to make some brackets to bolt it to, but it saves me from cutting my dash. I will try to get a pic or a link.
I've always had good luck with my Sunpro tachs, used them a few times, one of them I gave to my dad and its in his Van, after 17 years it still works great!