I can't actually take credit for the idea. The guy that used to host the "trucks" show on Spike TV gave that tip years ago and I've used it since.
OK, after perusing some pics of 1966 interiors I get how installing a modern stereo might be seriously difficult in these trucks WITHOUT cutting the dash. I guess I'll have to use some serious ingenuity to make something look good and still keep the dash stock. For all i know, it may already have been chopped. If so I'll just work with what I get.