If built right an automatic can handle a ton of torque and power. Lots of fuel cars are running high HP with automatics, and both PG and TH400. In an extremely high performance engine an automatic built to handle that much power is not something I'd want to drive a lot on the street. They require very high stall converters to handle the big cams and HP, and build a lot of heat.
If you don't mind some of the things required to run an automatic, and aren't planning on a lot of heavy foot driving, they aren't a bad choice. I'd go with the TH400 for mostly street use, as the extra gear will work better. Dave Roessler sells several versions that will handle whatever you throw at them. He even sells a newly manufactured SFI approved housing that's way more bulletproof than a stock housing. His race ready TH400 sell from $1300-$3700. He sells a street/strip TH400 also, but you'd need to call him and see if he thinks it can handle the HP/torque you plan for it.
Rossler Transmissions

 

Hi, thanks for your post. Sure I will contact him. I will also install an oil cooler to help the transmission oil temperature as low as possible. What about gear ratios and rear end ratio. As I said on older posts, I am planning for ford 9 inch.

From where i need to start working the ratios? 18 inch wheels at the rear.
Thanks

 

Picking an optimum rear gear ratio would be guess work right now. You need to nail down engine tranny combo first.

 

Yes, and you also need to decide what your driving needs will be for this car. If it will just be around town, then a lower gear will work, but if you want to be able to cruise on the freeway occasionally you'll want a taller gear. You also need to decide on the rear tire height, which will affect your engine rpm while driving.
I have 3.73 in both of my cars, but because of the different tire diameters (27.75" and 30") the engine rpm on each is different at the same cruise speed. Once you've chosen an engine and trans, then you can decide the rear tire/wheel combo and pick a ratio to match it all.

 

Where are you at right now as far as this build goes? What kind shape was/is the car in. Sure would be nice to have some pics to go in this thread.

 

I live on a small island so no highways and long roads as I see on the TV The car is intended to be used around town and for sunday shows with our club... Maybe one off overland (if someday I finish this project)...
I am tubbing the rear (DSE minitub kit) to accomodate 315 35 18. The tire diameter will be approx 26.5 inches.
When I was talking to the engine builder, for a 572 he recommended a ford 9 inch with gear ratio of 3.55. And he also told me to start first the rear ratio working the way up to the engine.
The 3.73 you have is on a 9 inch or chevy 12 or 10 bolt? What you think about 3.55?

First pic is the car as both Second pic is the car right now

 

I think the 3.55 is an excellent choice, as would a 3.73 be if it's mostly around town. But I'd stick with the 3.55 if you're going to 26" tires, as the engine will be happier.
My 3.73 gears are in two cars. One is my '46 Austin sedan with 327 SBC, and a narrowed Pontiac axle. The other is my '63 Falcon with 464 BBC, and an 8.8" Ford axle. The 8.8" Ford is a almost identical copy of the 12 bolt Chevy, except for 3.25" axle tubes, and 31 spline stock axles vs. 30 spline for the 12 bolt Chevy.

 

I am looking for an 8.8 for my truck right now. I agree they are a knock off 12bolt. They can be had with disc's for cheap and usually have a posi and decent gears. The Mustang guys can have theirs, the ones from a Exploder or F150 are dime a dozen.

 

Therefore I will stick with 3.55 for the ford 9". I will order it soon.

But first I need to work out by how much I need to narrow the new rear end. I will use the same existing perches location and narrow it from outboard of the perches ie:- reduce the distance from the perches to the axle tube flange. The stock dimension on my 10 bolt is 2 and 3/8" Maybe I can reduce it by 1" on both sides. So gaining 2 inches in total length. Is this how they normally do it? Or there is an alternative way? Thanks

 

How and where they make the cuts usually isn't based on spring perch location, as rarely are the spring perches in the correct place to line up with the car they're going into. A axle like a 9" Ford doesn't have a cast center section, so they usually cut the tubes right at the original weld, either at the center, or at the axle flange. Then they narrow it to whatever width is needed and weld it back together.
If you plan to narrow it yourself you need a jig to hold the axles and flanges true, or risk warping the tubes, and misaligning the axles to the third member. As cast third member is a bit easier to keep true, as the factory plug welds can be removed, and then the tubes pulled. Then pressed back in and plug welded, then perimeter welded too.