The Omega and Vectra/Cavalier/Calibra came with large cast iron exhaust headers that are entirely unsuitable for an MGB. New exhaust headers need to be made from scratch which might at first seem daunting but in actual fact was relatively straight forward but incredibly time consuming. You’ll need to buy a pair of flanges, some stainless steel tube and various 90 degree bends (look for dairy pipe and weld elbows). These are easily available from several sellers on ebay. Once you have these, you’ll notice that the exhaust outputs are oval shape whilst your pipe is round. You’ll need to cut about 3 inches of pipe and then distort one end of it whilst not allowing the other end to distort. This is easier said than done especially with stainless steel but perseverance and brute force will prevail. Once you have the basic shape, you’ll need to cut tack weld then into the header flange and cut them down as short as possible whilst still keeping the round end round.
Now, with the engine & steering column in the car, you should attach some metal strip or wire to the engine such that it follows the shape of the steering column. This is so that you can remove the engine from the bay but you still have a reference with regard to available space. This means you can design & manufacture your exhaust header whilst the engine is out of the engine bay. As long as it fits within your reference guide, it will fit in the engine bay.
The drivers side header will take a long time to get right. The passenger side one is simple in comparison. If you do it right, you will be able to route your exhaust inside the chassis rails so you won’t have to chop any bodywork. I’m no expert at exhaust design, but there’s no sharp bends and they look like they should flow gas relatively well.