First, what diesel engine? If a Perkins skip it, the Izusu is loud, reg Dodge Cummings too big, Powerstroke way too big, 6.2's blow heads.
Toyota's, H and B, and 2B's are not parts supported, 2H's are very good but hard to find, 3B's are very good but you may not like the power, HZ/HD-T's are very good but you must change to Saginaw or 60 series steering. PZ's same deal and parts are a bitch (5 cylinder)
second, do not go to CARB, quietly get the fuel type converted to "D" at a local registration site. Get out of an emissions county.
What size tires/gears is he running? A 3B is a great, torquey little engine, a solid 4-cylinder industrial engine used by Toyota on the 40, 60 and 70 series. It doesn't have a lot of HP, but that can be fixed with a turbo. Mileage is pretty good, too.
I'd consider the swap, depending on the condition of the engine and the expertise of whoever is doing the conversion. I would also take into account the type of wheeling you do, and how the truck's gearing will become more important than ever with a diesel engine. Remember, they like to run at low RPM's and have a much flatter powerband. If you're running 38" tires, I think a 3B will have a tough time keeping up without a turbo.
Do a search for Geoff Walsh Engine parts and you wil be amazed at how cheap rebuild kits and new heads for 3B's are! I know a source for 12v EDIC's, used but tested good. Next step would be a 12v starter (maybe 2LT mini truck winding) and then the Alternator. You do not want to mix 12v and 24v in your truck, all of one or the other. 3B's turbo'd are hot little items but a stock 3B is gutless for what we merican's think. You will also need the tranny input shaft of whole unit (21 spline not 10 spline like gas 2F's etc.).
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