Ok, all the carb 1600 & 1800 heads are exactly the same, so swapping will give you no benefit. You can however fit a 1.8 GTI cam which will give you a little more HP.
The 1600 block is exactly the same as the 1800 block (it even has 1.8 stamped on it), but the crank, rods and possibly pistons (according to ETKA, but you know how confusing it is!) are different to bring the CC down to 1600.
The 1.8 carb block is exactly the same as the 1.8 GTI block, so it doesn't matter which you go for. Personally I'd get one from an automatic GL as they are less likly to have been owned by a boy racer and thrashed, but the 1.8 lump is pretty much bomb proof anyway

The GTI head has bigger valves so fitting one to your carbed lump will give you a little more HP, but you have to figure out a way to block the injector holes. (one way is to leave the old injectors in) The other issue is that the GTI head will not have the coolant feed for the inlet manifold, so you may suffer from carb icing, and the 2e2 autochoke & waxstat will not function correctly.
You could fit a 2.0 2e block from a MK3 GTI or Passat for a nice boost in torque, paired with a flowed GTI head you can get some decent HP gains too, and even with the std carb head should give you a noticable boost.
You need to do 2 things to make it fit though, you have to block the breather hole off on the front, (I'm not sure where the carb fuel pum would go, there may be another plate?) and then swap the gear from the 2e dizzy onto your carb dizzy, then get an adaptor plate to make it fit in the hole. Both these items can be pruchased from TSR I believe

Hope this clears things up
