The FWD 5.3 W-body car's were completely slapped together to try and patch a "sports car" together with no budget and no time. The existing LS based engines share almost nothing in common with the LS4, other than displacement. The block is unique: different bell housing pattern, different timing cover, different intake (both size and orientation), starter mounted to the transmission.
Aside from the fact that they are a NIGHTMARE to work on, the lack of space in the W-body engine bay required the use of the already fragile 4T65E-HD. That transmission was already weak behind the 240/280 that the 3800 S/C put out. The 5300 was rated at 303/326... you do the math. The SS cars are absolute DOGS in 1st and 2nd gear because GM had to program in copious amounts of torque management to keep the transmission in one piece. They should have figured out a way to fit the 4T80E, which ITSELF was only used behind 300/295 in the higher end Cadillac models.
*EDIT* I was curious so I did some looking... there's actually a HUGE aftermarket that supports swapping the 4T65 for a 4T80. There's a custom oil pan which allows an engine mounted starter, a custom mount kit to locate and clearance the transmission, and countless guides about the (minor) trimming needed on the block and transmission case. Even better, the 4T80 can run off a stock GM TCM with the proper flash, so it operates like OE.