yeah the '65 GT or the '66 cluster should fit. although for some reason, i don't remember why, or if i'm just imagining it, i think there's some small minor issue with the '66 cluster in a '65... correct if if i'm wrong.
I actually just did this. I took 66 gauges and put them in a 65 and bought a 65 GT style bezel for them. I had to drill down a little notch in the opening to fit the 66 gauges in, but it wasn't that big of a notch only like 1/4".
Here's the gauge support & Autometer version...
I did NOT have to notch the dash. You can simply trim a little off the back of the plastic bezel - it doesn't show when using the Autometer speedo. The gauge support is available thru MDJay, CJPont, and several other places...