no, definitely Gen-III Chevy, not Northstar.
The Northstar is absolutely huge and was designed for front-wheel-drive (yes, really), so doesn't readily adapt to correct-wheel-drive. I did a Northstar installation back in 1996 for a company I can't name on here, there were no rwd Northstars available then, so I had to design a bellhousing for them. I believe that there is a Northstar now available with rwd, so at least the bits are available. Cadillac STS ?
Not aware of any other rwd Northstar conversions around. Certainly all the fwd donor vehicles need to be hooked up to the 4T80 transverse gearbox for the electrics to work - much more difficult than with an LS1 (at least an early one anyway)
The "original" manifolds for the Omega V8 were tubular - you have the correct pics, but these may have been low-volume pre-production items destined to be replaced with cast for series production. Any way up, I have no fear that current after-market "tight-tuck" manifolds can be made to work with the Omegas steering box.
The clearance pictured isn't that bad. In the region of the manifolds, the engine movement is "up/down" and not "side-to-side". 3/4 inch is good enough for clearance under all circumstances and to slide a heatshield in.