Re: 1966 289 build
I don't think you will have too much trouble making 300+ hp with that combo. Shelby made 306 hp with a similar setup back in 66.
The 66 289 heads were not terrible since they didn't have thermactor bosses in the exhaust ports. Just clean them up a little and port match them to the gaskets. One of the things we used to do back in the day was replace the valves with 1.90 intakes and 1.60 exhausts. TRW and Manley used to make them. I think they still do. One thing you definitely should do is install screw in studs.
Shelby used 11 to 1 compression pistons but that's a little much with today's gas. Shoot for 10 to 1.
I personally prefer a dual plane intake to a single plane. Also, Shelby used a 715 cfm vacuum secondary Holley back then. THAT carb is magic on a 289.
If you really want to keep it old school Comp Cams makes an updated version of the Shelby cam.
I had one running 12.0s in th 80's in a 3000lb car basically running an engine like this.
1966 Falcon, 347 stroker and C-4