Hi Adam,
This is the second time I've responded (computer crashed as I sent the first reply!

) so here we go again!.......deja-vu for me, new info for you.... :lol:
Just my thoughts and experiences, but maybe worth something to you??
Unless you are going to be using the motor mainly for off-road, I would say that the BFG MT's might be a bot too aggressive for you?

The tread pattern causes quite a bit of road noise and vibration. This can be felt not only through the steering and suspension, but also as a drone through the whole body, getting louder at different speeds.... :(
If you are going to Mexico (a long trip - especially by UK standards!!) then you might find the MT's get quite annoying and male the journey less enjoyable!
They will also give you a drop in MPG - again due to the aggressive tread, but MPG doesn't seem to be as much of an issue as here in blighty because your fuel prices seem to be a 'little' bit more realisitic! :lol:
If you can, I'd recommend BFG AT's like Kevin has mentioned...
These are a FANTASTIC tyre for all purposes: Very good tarmac properties, good braking and handling in the dry and wet and also good MPG compared to most other similar treaded makes (like G/Grabber etc..)
The tread seems to last forever, and you'll clock up some SERIOUS mileage before wearing these boots out....
Off road characteristics are a
little less favourable than the MT's, but in general they are a VERY competent off-road tyre and what you lose in slightly less off-road ability, you
more than gain in aboslutley EVERY other respect....
I would see if you can find someone with MT's on their motor before you buy them for yourself - try them out on someone elses motor and maybe see for yourself? AT's will happily get you through most muddy, snowy and watery situations.....it is only at the extreme end that the MT's come into their own, but at the sacrifice of a lot of other aspects.
About the size - 30's or 235's will be pretty much a straight fit with only minimal work to get them to fit (dependant on the offset of the wheels and where the torsion bars are set....)
Personally I would say that 31's are the perfect sized tyre for an L300. I had these on the MM before heading over to 33" territory! The 31's were a superb blend of size/grip/handling/perfomance etc....
A minescule amount of acceleration was lost, but a marginally more comfortable cruising speed:revs was gained. The loss of acceleration wasn't an issue - if I wanted a quick getaway, I wouldn't be driving a 2+ton Jap Tank! :lol:
When planning the scheme and details for the Mud Monkey, the ONLY sized tyre considered was 31" NOTHING else!
To me (and many others in the UK) 31" is the automatic choice for a semi-standard '300. It's only when you get over 4" of lift that you'd think of anything else.
Looking at your timescale of when you are looking to buy - I'd be happy to shop around with you when I'm over - help you find the bset size, fit them, modify the arches for you etc.....(even do the torsion bars

)
Just let me know??
Regarding the 31's and your existing wheels, I have some possible solutions for that too....but will chat about that on the phone sometime soon.
Best of luck and regards for now,
Bruce.