You will be just fine with your 8". That's what I run with my 5.0 (also with GT40 heads and other goodies). Mine is not the original 8" that came with the car as mine was also an original straight 6. My 8" has the reinforced webbing on it and was an open 3.00. I've since upgraded it to a limited slip 3.55.
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I would just find a used 8". I sold mine a few years ago when I did the Explorer swap for $110. It was a 2.80 peg leg, but it would be a perfectly workable solution on a budget. Served me well for 20+ years before I got rid of it.
Here's the thread on my adventure with the Explorer swap:
www.vintage-mustang.com
9" rear ends may be plentiful, but in the width for a 65-66, ready to run, they are expensive. It's surely the most bullet proof solution, but it is also overkill in this application. Without knowing the OP's budget, I can't in good conscience recommend a 9 as the only way to go.
For what you get in the end, an Explorer swap is great value for the money. It does require some fabrication, and by the time it's done is often more expensive than most report. I'm glad I did it and would to it again. That said, for my daughter's 66 that will have nowhere near the power that mine does, she's sticking with the stock 2.80 peg leg 8", which will be just fine.
Here's the thread on my adventure with the Explorer swap:
What does it really cost to swap an Explorer 8.8? I can...
You hear about the Explorer 8.8 swap being the most affordable path to having a super-strong rear end with disc brakes and steep gears (3.73) while driving both wheels. For context, I was coming from the stock 8 inch, 2.80 gear open diff axle with drum brakes. This swap is often described as...
www.vintage-mustang.com
My 66 started as a 200, 3spd. Now a 331 stroker C-4, 8" 3.55 posi. I also changed the front to 5 lug and manual disc brakes. When I changed housing from the 7.25 to the 8" I found it on ebay for a 66 Stang. They have the narrow housing ends. Because I was going to rebuild it anyway I just sent it to Currie, had it straighten, cleaned powder coated, new brake assemblies, and forged axles made for it. But that's just what I did.
Get the right 65-66 housing and just rebuild it and stick it in there.
Get the right 65-66 housing and just rebuild it and stick it in there.
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8" is just fine....and for perspective, in other countries where the 8s and 9s aren't so common, plenty of people have made do with the 7.25s(its only in the US that they have a bad rep because of their much stronger big brothers) behind v8s in drag racing applications:
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13 second 1/4 mile times with a 2 year life span of drag racing....not shabby for a heavy 4 door Falcon. I am not suggesting keeping the 7.25 when you have better options...just that people discount them out of hand for mild builds when they would likely hold up fine for a 250HP engine if they aren't beat on...lord knows I have pushed more power and torque than that through a 6.7" axle myself in my old RA29 Celica
13 second 1/4 mile times with a 2 year life span of drag racing....not shabby for a heavy 4 door Falcon. I am not suggesting keeping the 7.25 when you have better options...just that people discount them out of hand for mild builds when they would likely hold up fine for a 250HP engine if they aren't beat on...lord knows I have pushed more power and torque than that through a 6.7" axle myself in my old RA29 Celica
That's because she was left speechless. 
Now if you'd had a 9"er, you could have really put the power to her.That's because she was left speechless.
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