Here is what I did....it was a lot of work but it fits in well...still need to get to the upholstery shop to get the dash covered.
I have to take a better finished photo after the fiberglass, but I attached one I took from when I first drove it.
I have 74 K10 pickup (Non AC, Automatic 350)
I am looking for the heater core tank strap that is missing in the photo below. I have the strap for the lower side of the core but for some reason when I took this apart to restore it the top strap was missing. I have been looking all over the web...