X3 on inspection plate being best way to tell on the trans.
I dont think someone who bought twins would put only a converter in there so its safe to assume that:
Stock bare converter=stock trans.
Painted converter=built trans of some sort.
Suncoast doesn't paint their converters if I recall? So I'd be looking for a drain plug in the converter.
Why do you believe or makes you believe it has one, a converter, to begin with? If it does I would tend to believe some work has been done to the transmission. I wouldn't think someone who had the funds to do twins would pull the transmission JUST to put a converter in it without additional work. How much? I dunno but depending on what you want to do with the truck it might be enough. Only the original owner will know unless you take it apart and find out.
I'm also on the side of the fence that believes nothing has been done internally to the motor so I'd be overcareful about turning the wick up to full potential of the twin CP3's.
Nice truck nonetheless. Would be nice to talk to the original owner. But if he lost it or had to sell it because of the economy he might not want to hear from you either. He might be scorned after spending all that coin and NOT be able to enjoy.