66 GT350H - C4 to T-10 Swap

Sounds like you are planning to drive it more. Value change? Maybe, but that's only if you're selling, and, if you keep boxed up all the parts from the automatic installation, it would be worth even more. Most performance buyed would prefer the stick.
Might I recommend a 5 speed? This way would cost more, but, the next thing might be to change your rear end gearing if gas still goes up. With the 5 speed, you're already there. Small problem: you'll need a slightly shorter drive shaft. Might not want to shorten than genuine one.
Morw 5 speed installation info at www.vintage-mustang.com

 

I second the 5 spd recomendation. Cruising at 70 and 2200rpm takes stress of the engine and the driver. Besides if someone ask if it has a alum. box, you can just smile and say yes!

 

I did the T-5 swap on my 66 non Hertz automatic and I don't regret it. I kept all of the original parts but don't plan to go back. In my research, I read that the Tremec would require modification to the tunnel on a 65 or 66 so I went with a new "motorsports" T-5. I had a new, stronger, driveshaft made to go with it. Consider what you are going to do with the car. If you are going to put the car on the track a lot, you may want to consider a toploader or even the T-10 you mentioned. If you want a street friendly car, you can't beat the T-5. No cutting, no welding. I had mine on the interstate a couple of weeks ago and was able to run the left lane and pass everything that needed to be passed without even breaking a sweat. I love toploaders, but running modern roads with a 3.89 rear and no overdrive is only good for me in small doses.