What size turbo should I buy for a 5.6L engine?

I'm going to put a turbo onto a 5.6L 345CI International Harvester V8. This is a fairly industrial, slow spinning engine from the factory. It currently has th spreadbore 4bbl on it, and as far as the engine itself, is more or less stock. I HAVE converted the fuel system to an LPG setup. currently it is a standard regulator/mixer setup off a snorkel on the carb. Once I get the turbo system on the engine, I will convert the LPG system to a BLOS LPG mixer with European regulator setup for the blow through turbo.

So what I am looking for is what size (preferably with part numbers, and what vehicle they might be salvaged from) of turbo impeller I should be looking for.