both are possible.....
regarding blowing the V6, yes it's possible. My early measurements mean that it's likely that an Eaton M45 or M62 will fit inbetween the cam covers, but would of course need a special manifold building.
the V6 has a reputation for cooling problems, but personally I think that this is suspect. Cooling can be compromised by a high "normal running temperature", very often variable from car to car. The basic cooling system seems well designed to me, and should easily cope with +50% horsepower.
regarding V8's - I would have previously said that fitting a V8 would be beset with steering box clearance issues. BUT, there's an LS1-powered car currently on e-bay or Pistonheads, so it must be possible. I'm currently scoping this for a potential conversion.
LS and T56 will fit under the bonnet, but it wouldn't be easy. Expect £2-3K to buy the engine & box, plus the same to fit. And there'll be electrical and braking issues, too. The £5k for that particular car may well be the best bet, although it certainly doesn't have more effective brakes than standard, so you'd need to put aside funds for improvements.
Omega B is based on the V-car platform, which is also Holden Commodore, VX Monaro and Pontiac GTO.