If, as is widely believed, the GM LS1/2/3 won't fit, I've a couple of suggestions:
1. Mercedes M113. 4.3 - 5.5 plus supercharger potential. Fits in a C Class, which has a steering box but auto only. About £2000 for a rusty E430
An older, and therefore cheaper alternative, the M119 4.2 - 5.0, as found in the W140 S Class, but also fitted in the W 124 E Class, unloved about £1000
280+ bhp.
2a. BMW M62B35/M62B44 as found in the 5 Series, 96-98, available with a manual box, again £1-2000 for a complete car. 280 bhp.
2b. BMW M60B35/M60B40 as found in the earlier 7 Series and 5 Series, 92-96. sub £1000. 275 bhp
2c. BMW M62TUB44, 5 Series etc, from 98. Canbus, so perhaps not as straightforward
280+bhp
2d. BMW S62B5, from the M5. Most expensive choice from this lot. 350+ bhp
There you have it
My money would be on the Merc lump as the cars they were fitted to have steering box set ups, so packaging might be easier
then again, it wouldn't be the first time that someone dropped a BMW lump in an Omega, would it
Off down the shops to stock up on popcorn...
P.S. Chris, I did say you might not like the solution