The easiest upgrade is the short TH350. It will require the removal of the front lip of the trans cross member to clear the pan, but everything else is simple.
The 200 should be able to use the stock crossmember and drive shaft, but will require a tv cable and a lock-up wiring kit with the convertor or a non-look-up converter.
The 700 requires the same as a 200 plus a new trans crossmember and a shorter drive shaft.
The 350 is definitely the easiest upgrade and you may be able to trade your PG for a 350 as in most places the PG is harder to find.
The real question is what do you mean by a better transmission? The PG is good reliable unit, a 350 will give a little more performance and either of the R4s will gain some performance and with the OD maybe better economy. It is a matter of how much do you want to spend, how mechanically adept are you and what other mods are in your plan.
