If im looking at a b body with around 540 hp and 600 ft lbs MAX (at the flywheel), street tires, and 3.73 gears, what chassis stiffening would be necessary? Im thinking of at least torque boxes and possibly subframe connectors, would anything else be needed? Also, other than rear end (probably putting a dana in) what else would i need to strengthen my car?

Put in all the boxes and floor plates, weld in subframe connectors, full tube thru the floor are the best IMHO, pull the K frame weld that up, gusset the steering mount, box in the motor mount towers, lower rad support, and side firewall braces are nice additions, improve the welds thru out the unibody, shock towers, rockers, rear rails/floor, transmission crossmember...

Listen to what Dave Dudek says about chassis stiffening....
It’s about 15 minutes in.
https://uncletonysgarage.com/2018/08/13/podcast/episode-4-mission-impossible/

Go with through-floor frame stiffeners, I sure wish I did. I'm not impressed with the laser cut rigidity.

If you have the car apart you should do it all. Ideal situation is if you have the bare unibody on a spit so you can spin it around and do justice to the welds. Laying on your back putting torque boxes and subframe connectors in is no fun. Getting the car up on a hoist will ease the pain some but being able to roll the car over is the way to go if possible. Of course, the car has to be stripped way down to do that and that takes a lot of time and money.

As a back handed compliment, I would have thought you knew better.

I did, but got the whole kit and went along with it.

From my limited experience with these cars, torque boxes, subframe connectors and a welded & reinforced K-member work wonders. Under fender braces & under radiator support significantly helps stabilize the front end. Add a custom Monte Carlo bar to finish it off and further eliminate cowl shake.

Welding up the rest of the car's unibody welds as mentioned, helps too. It all adds up, and it really helps if you start with a relatively rust free low mileage unbeaten car.

Some upgraded control arms, HD tie rods & sleeves, sway bars bigger torsion bars, stiffer springs and quality shocks will make that stiffer chassis pay off.

Beef up your driveshaft, u-joints and attachments to the rear end and transmission.

If you don't live where it freezes hard run some Nitto drag radials out back to hook it up on the street. If you plan on running common street radials, you can count on traction problems.

If you are running an automatic, spend $ on a quality torque converter matched to your combination. Better yet, put in an overdrive transmission and EFI it.

I clicked on the link. Nothing about chassis stiffening from 15.00 to 24.00.
If I were a drag race guy, that would have been time well spent.

I guess you don't plan to race it but I can say my 6 point roll bar that I welded over the unibody chassis stiffened my car the most. It has frame connectors also and I had to have at least a 6 point rollbar when running under 11.50. And I would put it in any car I race anyway because it basically ties into the front and rear unibody frames and really stiffened my car a lot. I did not want a full cage which really stiffens the car because its mostly a street car and I did not want more bars to climb over. But I noticed that the bar seemed to stiffen the car even more then the frame connectors did. Ron

The chassis stuff was somewhere around the 40 minute mark. 318 TBs, 318 springs, no snubber, no subframe connectors. G70 tires being the limitation.