I actually have a 3.0L/181 c.i. GM inline four. Of course, it's in my Bayliner!
It's a tough engine with gobs of low-end torque. Another strength is that it uses a timing gear instead of a timing chain- good at high rpm's. They do work hard in a marine application, though. This means that they can be sensitive to overly advanced timing or low octane fuel.
Now, putting one in an automotive application would be tricky since you don't really have the manifolds, etc. for it. I doubt a forklift is going to have really high-flowing manifolds, either. Fuel injection is rumored to be in the works from Mercruiser, but all 3.0Ls are still carbureted for now. Heck, I still have points ignition and a 2-bbl. Rochester on mine. I can't easily upgrade to electronic ignition due to the Electronic Shift Assist. I haven't had a problem with my points and figure they're cheaper than tearing up the gearing in my outdrive- $$$!
My recollection is that this engine traces its roots back to the Chevy Nova II back in the '60s, particularly for the South American Market. It's been around a long time and has a rock-solid track record.
I hope this sheds a little light on the subject... :)