I've never heard of an Isuzu compressor for a square body. Usually when you talk about A/C upgrades people will say "Sanden", but that is a brand and not a specific model. The stock R-4 has larger diameter and less length than almost any alternative compressor, so moving away from it requires new mounts ("R" means "radial", so there are 4 pistons all around the crank, which is why it has such dimensions.) If you search, you can find popular models for a conversion, based on the reliability of those units. The drawback is that none of them offer the same performance as the Delco R-4, because the stock compressor has more displacement. Likewise, it's popular to switch to a parallel flow condensor instead of the stock tube-and-fin style, but those are also smaller. The simplest approach is a NEW AC Delco R-4 compressor, as they are reported to be quieter and more reliable than the older models were. "Best" is a matter of opinion. No matter what, the performance of the radiator fan is critical, since you can't remove any more heat from the vehicle exterior than you remove from the condensor.
Use the search feature here, lots of discussion.
