What should an intake porting job cost for a 99 GT intake? I dont see a point in going from 65mm to 70mm so I was considering porting the intake and going to 85mm.

 

1. You can't port the lower intake, its plastic.
2. Upper intake porting will likely flow as well or better than most aftermarket plenums, so your right about that
3. 70mm is the biggest you should run on a NA stang, besides the stock plenum won't flow any better with any plenum size over 70mm, since thats about the size of the plenum itself.
4. 75mm is the biggest you can find for a non-race setup, and its not likely you would ever find something as large as an 85.
5. 5mm difference is bigger than what you think it is

 

interesting. My last "real" car was a 1967 Camaro that I restored so all this is new to me. Plastic? geez. It was so easy to get my 67 into the high 12's. 350, 64cc 1.94 heads ported and polished, ported and polished Edelbrock intake, fat cam, and a Saginaw 4spd. I ran high 12's all day. Only spend about $7000 including the car, haha.

 

Yep times have changed