I had an 83 Pup with the horrible rattle problem. I got mostly rid of it by just disconnecting the vacuum advance when I got just really sick of it one day, so I suspect the distributor was the source of my problem. This has been quite a while ago, and I didn't put much into solving the problem since the truck wasn't much anyway. Pulling off the vacuum line got rid of the rattle except under really hard acceleration, so from what I've read here I'd guess the mechanical advance was pretty well stuck. Too bad I don't still have that truck.

(Tho I saw it randomly about 60 miles from where I last had it sitting for sale in some farmer's barnyard.) One question to add though, did you use a points distributor or electronic? I had electronic and always suspected I needed to replace the module in the distributor, got so used to points going out all the time that I figured that sucker must go out too. Not like Isuzu to replace one problem without creating another.
There are a ton of sources for rattle in the 1.8 motor though. Not the least is the oil pump. Under hard acceleration with low oil level the pump can suck in some air when the oil sloshes back in the pan. If the oil pickup tube is out of place I suppose it could do the same. Oil pump rattle should be familiar though, it is the noise you hear the first time you start up the motor after changin the oil and filter. Timing chain or tensioner wear will cause a constant rattle coming from the top end, and valves out of adjustment will do the same. Just