Nope Gibbo you don't need a kombi tranny, you can use a swing axle or IRS (which you have) box but in the flywheel (either 210 or 215mm) you need a brass bushing pressed into the centre so it can mate to the beetle box input shaft. I had this done down at Crankshaft Engineering when they balanced all my type IV bits, lightened the flywheel slightly etc etc.
1700 and 1800 type IVs have a 66mm stroke but the 2L have a 71mm stroke. You can mix and match though - I used an 1800 case with 2L crank/rods with the 94.5mm (0.5mm oversize) pistons....plus 2L heads but with the larger 1800 valves. Type IVs are generally more expensive to build than type Is but longevity and torque are things which are in their favour.
See this site (technical articles section) for lots of detail.
http://www.tunacan.net/t4/index.shtml
Alan