It's going back a bit now, so the job is not fresh in my mind, but from what I remember:
The head studs were no problem as you can access them from above, you may need a 'wobble extension' to get at one or two of them at a slight angle around the manifold primaries
The 4-2 to cat bolts were more difficult and if these have rusted and don't want to come off then this is where you'll have your fun - getting leverage with a breaker bar and assortment of extensions while lying flat under the car with not much room to move can be pretty frustrating
The cat to b-pipe bolts are a bit easier but the top one (IIRC) can tax you a bit
You can disconnect the O2 sensor without removing the sensor from the 2-1 section, there's a connector somewhere up towards the bulkhead. This allows you to remove the sensor with the exhaust part off the car, which may be a bit more 'controlled'.
I had to raise the rear of the car to get enough room to swing the Toda out as it's a one piece design, but you won't have this problem swapping OEM with Spoon as they're both two-piece.
You need to drop the ARB out of the way to give you room to get the 4-2 through into the space behind the engine. The OEM one with heatshield will be worse than the Spoon part as it's much bulkier. Tbh I can't remember whether I took the heatshield off, probably did.
I personally wouldn't bother doing the job in two halves, I'd do the job once and do it all.
A bit off topic but I personally wouldn't use the Spoon 2-1 part, I'd just change the 4-2 and keep the OEM 2-1/cat.
My 2p, hope it helps a bit