I'm sure this has been beat to death and I apologize for it.
I have a 450-500rwhp sn95 mustang. 3,000lb car with a C4, transbrake and 28" tires.
I have 2 questions.
1. What gear? 3.55 or 3.27 gears?

2. Second question I'm looking at a 72mm turbo a d what size A/R? 6,200rpm trap speed. 6,000rpm shift points. I want it to spool as fast as possible and hold it to 6k rpm. .63, .71, .81??

Background story on the car. E7TE heads with tfs valve springs, 1.72 roller rockers, 90333pt1 head gaskets, head studs, explorer intake with 70mm throttle body and stock cam.
C4 with transbrake.

Tanks in advance!!

 

Rear gear is calculated based on the MPH target and max RPM of the engine including any converter slip.

For example
https://spicerparts.com/calculators/transmission-ratio-rpm-calculator

3.55 gear, 28" tire, 6000rpm, with NO SLIP can achieve approx 140MPH

So if the car has enough power to go 150MPH+ but the max RPM of the engine is 6000 you will never achieve the right MPH at the end of the 1/4 mile.

Same goes for 1/2 mile and full mile racing. You must calculate the desired MPH and use the minimum gear/tire to get there. Then remember to factor in converter slip, tire wear and tire size changes over time.
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As to turbo sizing. The right way to do this is first determine the application. For example in drag racing, if you can trailer the car to the track. And it has nitrous, trans brake, CO2 control, 2-step launch control, etc... Then you will choose a turbo that will run up into the center of it's island for the entire race. This type of turbo will spool slowly without the help of T-brake, 2-step, nitrous but since its never run on the street that won't matter.

For second example say we have a car without all that stuff that does drive on the street. You want to light the turbo and tires up from a roll easy, and you want it to have good cold start performance. This type of engine will use a cast aluminum piston for good cold behavior. Try not to mix and match parts. Forged aluminum pistons require warm up time so a turbo sized for street and cold start performance will not be useful since you can't run a forged piston hard in the first 30-45minutes of starting the car or it will scuff and wear out the cylinders and damage the skirts over time.
Anyways. The type of turbo you choose for this is one where it will just be running off the end / edge of the compressor map at max output. This will ensure you've selected a minimalize compressor wheel mass which will maximize the spool character without any nitrous or 2-step tricks.

For example here is my turbo for my daily driver, which offers great cold start performance and approx 800rwhp max for street activity. I can light them up from a roll in 2nd gear at any speed 2nd can offer me and especially in 1st of course.
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The red dots showing how it approaches the end of the compressor map, falling off just as the engine red lines, is IDEAL for a street car that is a daily driver. That is how the OEM sizes factory turbo chargers, just to fall off at max performance will offer the best spool character. This is where your turbo selection starts, not in the exhaust side.

The exhaust side only needs to support the power and exhaust configuration. I recommend using match bot like I did above,
https://www.borgwarner.com/aftermarket/boosting-technologies/performance-turbochargers/matchbot

Using match bot to first select the right compressor map for your application, THEN match the turbine behavior using match bot to feed the expansion ratio and so forth, read the instructions and good luck.