If you are going with an army that consists mostly of infantry, Infiltration is the best. It also fits your plan well as it gets its biggest bonuses around 1943, which is when you would likely be facing the US rather than South and Central American countries.
As armor is mostly pointless in South and Central America due to the large provinces, slow movement speed and many bottleneck provinces, Blitzkrieg isn't really the best doctrine.
I doubt you will have the IC to make good use of the Superior Firepower doctrine until far later than it would really matter.
This leaves the choice between Infiltration (the other Grand Battle Plan doctrines are simply not that great) and Human Wave. Peron is pretty good at researching both of those branches. If you think you will start facing tough opponents before 1943, go with Human Wave. If not, go with Infiltration.
The main downside to Infiltration, other than how late the bonuses kick in, is its reliance on infantry and artillery (No cost reductions on anything other than INF and ART, and cost increases on ARM. Also, you need to attach artillery to units for the huge morale and organization bonus after 1946). Because of that, you armies will be really slow, especially against the US, which will have a lot of mechanized and motorized divisions.
Upside to Infiltration is that you can trade with Japan for the blueprints if you ally them.
