5.3 swap issues! Help!

Recently I've swapped a 2003 Chevy Avalanche 5.3l into my 1991 Nissan 240sx. The engine is backed by a 4l60e out of a 2006 2wd silverado. The only mods to the engine are the custom headers for clearance and a ls1 intake (not ls6), and a corvette ls1 fuel rail, and a corvette fuel filter. The pcm/ecu has a performance tune and the wiring harness has been thinned out by wait4me.

I've been having a lot of fuel related issues. At first the car wouldn't idle good, so we put in the walbro pump. Then it idled better, but wouldn't rev up or hold a rev. So we put the truck fuel rail on, idled and revved fine, but drove rough (we also had to remove the corvette fuel filter and replace it with an envoy one because of return line setup). So we checked out the corvette fuel rail and decided it was in the regulator, it was on a weekend so we just hollowed out the factory regulator. The car then ran okay, idled smooth and rev's great. With a snap-on pressure gauge, the rail was getting 60psi.
While driving the car, it doesn't like to shift into 4th or overdrive unless your slowly accelerating with hardly any throttle. If you punch it, it will downshift to 2nd, then third, then it chugs/bucks (runs rough) until it reaches a certain rpm then it just seems to sit there, almost like a rev limiter. It won't shift to 4th anything past 1/4 throttle.
My issue is with the car running rich up top, I've been told it should be around 44psi at the fuel rail, so we put the truck rail back on. It was getting 44psi, but the same driving problems still occured. The bottom end isn't too bad, it runs smooth in the bottom and majority of the mid range, but top end sucks and it seems to be running rich and it doesn't pull very hard.

We've tried just about everything, switching rails, two different regulators for the corvette rail, back and forth with no real gains. We have a ls1 MAP sensor in the ls1 intake.

With the Solus scanner hooked up, the 02 sensors bounce around and are doing thier job, the a/f ratio is 12.4, the tps sensor is working properly and it's not throwing any codes.

So, does anyone have any ideas what it could be? We're at the point of thinking they'res wrong with the tune and we need to send it back, but that's sort of a last resort.
Any help would be greatly appreciated and if anything isn't clear please ask.
Thanks in advance

Sounds to me like it's a tuning issue, assuming your fuel pressure is correct. Is it a return or returnless fuel system?

Are you running a speed sensor...VSS and is it hooked up. AFR shouldnt be 12.4 at idle or part throttle ...maybe WOT

All Trans. Wiring Is Hooked Up And The Fuel Lines Are The Stock 240 Lines (3/8 In 5/16 Out) The Vette Filter Was In Correct With All Gm Fittings--- But The Stock Truck Rail/regulator/injectors/return System Is In Now With Same Results

It has the return style system on it now, but we'd really prefer to run the corvette fuel rail with the either the proper pressure regulator or the hollowed out one, and the corvette fuel filter.
Would swapping to ls1 injectors (and redo-ing the pigtails obviously) help this issue? It wouldn't make sense to me though because it idles soo smooth and the bottom end isn't horrible, but I'm not sure at this point.
Thanks for the help so far guys