Problem being even if you look out for a particular F rating you'll normally also still have a choice of actual ratios.
There's nothing off the shelf that has quick low gears and a longer fourth and fifth, they're either close or wide ratio.
You can pick and choose gear set and diff but that's a big job stripping it down and I'd be getting it running in a car first and taking it back apart from that job if you're not happy afterwards.
So many other factors will affect it - where the power is, wheel size etc. really a low priority when there are other jobs in front of that one.
Engine conversion on the cards within the next year or two.. Staying small block because a big block engine in a Corsa b is far too heavy.. Now the 1800 16v engine I have decided to fit it roughly 120horse.. Currently in the car is a F10 or F13.. Definetly being replaced for something stronger with better ratios.. Currently the 1200 8v engine is doing 3500rpm at 70mph on the speedo.. Ideally I would like a box with quick 1,2, and 3 with fourth and fifth being longer for motorway use.. What boxes will do what I'm looking for?