Anybody know?

nothing, they are not compatible. Atleast the 00 fule rail isnt. The 4thgen injectors are side feed while the 00+ are top feed.

IIRC

00-01 injectors are top feed saturation and 95-99 are side feed peak and hold ,not going to happen id just get the 00 lower and aftermarket injectors and sell your setup.
You could get the 4th gen lower welded and machined to fit the 00 upper

Yeah, I know. But as far as inserting the 4th gen injectors and fuel rail into the 2k lower IM, what needs to be modified? Is it simply a matter of enlarging the diameter of the holes in the IM? Or what?

you would have to weld more metal in those spots and machine it like the 4th gen holes then weld and make tabs so the rail can be bolted on like the 4th gen because the 00-01 bolts on very different.
Much easier to weld and machine the top of the 4th gen manifold to fit the desired application

Okay, thanks, Krismax.

the 00 lower is alot less beefy and weighs almost 4lbs less than the 4th gen one id have the top welded on the 4th taped and plained

Anyone know if I will run into problems when running the 00 injectors with like a JWT or Technosquare ECU programmed for the 4th gen?

I have a JW ecu and 00 injectors no problem and the 00 injectors are fine.

Just tell JWT you had RC Engineering make you *CUSTOM* 290cc injectors, otherwise they won't believe 2000-2001s don't use 240cc injectors.