If you do a search through
http://car-part.com/ or look in a chilton's manual on Isuzu vehicles you will quickly find out which year the 2.3 came in the Pup and the Impulse.
I just did a complete USA search on car-part.com and according to them the 2.6 came in the 88-91 Pup, Trooper, and Amigo .... and the 2.3 came in the 86-92 Pup, Trooper, and Amigo, plus the 88-89 Impulse. The 2.3 carb intake and distributor/ignition module needed to put the whole thing together looks like it came in the 86-87 Pup and Trooper. But don't quote me - that's just what their data shows.
The 2.3 intake, distributor, and ignition module will bolt right on a 2.6, and the distributor/ignition conversion is a 2 wire connecton just like puting a newer electronic setup in a Luv's stock 1.8. You will want to get a fairly new 2.6 because some of the earlier ones had head problems.
An Offy 1.8 intake probably can be massaged to fit, but IMHO it shouldn't be necessary. While you are at it, I'd snag the cam out of the junk donor 2.3 and send it off to Delta Cams for a custom regrind for your 2.6 engine and then bolt on a decent set of headers. And I don't know for sure if the weber will bolt right on to the stock 2.3 intake, but I suspect not. But the good news is there are some great deals on used weber 38/38's on ebay that take the same rebuild kit as a 32/36, and will render a little more muscle out of your setup without costing you much noticable gas mileage difference.
A de-smogged 1.8 with a cam, headers, offy intake, electronic distributor/ignition, higher out put coil, and a weber 32/36 IMHO produces around 125-150 hp max (stock w/smog and A/C is 75 hp), depending on the cam, type of headers and exhaust, jetting in the carb, head work that has been done (milling) etc.
A stock 2.6 w/ smog and the A/C connected produces 125 hp - so with the right intake, a slightly bigger weber 38/38, a decent set of headers and exhaust, a higher output coil, a custom reground cam, a decent head, etc. - you should be more than happy with the results.
And if you throw in a 5 speed and rear-end from a newer Pup, you will be driving around a heck of a lot faster than before with a permanent smile.
Here's a link to a few more detailed threads here that go into just about everything about using the 2.3/2.6 hybrid engine in a Luv to get you started heading in the right direction:
http://forums.luvtruck.com/viewtopic.php?f=6&t=15987http://forums.luvtruck.com/viewtopic.php?f=5&t=15872 (scroll down a bit)
http://forums.luvtruck.com/viewtopic.php?f=5&t=15674