I'm in the middle of a 90 Amigo sas and engine swap. Here's what I know so far:
If you want to keep upgrades simple stay with IFS. If you want more capability off road than the IFS can handle prepare for an interesting project.
As far as engine swaps, see comments below about the GM V6 options.
For SAS:
Waggy Dana 44 or Toy solid axle. Passenger drop. Toy axle wms measurement is closer to Amigo stock. 77 waggy is 61.5" wms/wms, about 2.5" wider than stock.
I'm doing a sua with the waggy axle. If the axle is moved forward 3-4", uptravel is good to the point the ubolts (flipped) hit the frame. To get maximum travel and articulation with this setup you will need 12-14" shocks (meaning the top shock mount is close to the underside of the hood!), and lots of fender clearancing. The pumpkin will hit the oil filter on a 2.6 at this point but no problem with a Isuzu/GM V6 (2.8,3.1,3.4)engine. I believe this setup will give the lowest possible center of gravity.
There is a coil over conversion thread elsewhere on the internet if you want to go that way. I'm staying with leafs. Depending on how you will use the rig, spring choices range from waggy stock, YJ front lift from ProComp ($140/pair, shipped on Amazon)to Deavers to others. I choose the Deaver YJ front 4.5" lift. These are about 1.5" longer (at full flat) than the stock waggys or ProComps. With shackle swap (shackles on the rear of springs) set for about 1" clearance to the body mount/crossmember bracket, the Deaver springs move the axle CL about 3" forward. Rear shackles also mean the front hangers have to be mounted further forward, meaning a frame extension more than just adding an additional front crossmember. To get the spring eyes close to the same height (the way waggy engineers did it) the front eye centers need to mount about 5" below the bottom of the frame. If approach angle is critical to you this would not be the ideal setup. I'm not quite far enough along to determine if I can get away with castor correction shims or if the knuckles will have to be rotated. The other big issue becomes steering. End of pitman arm is well behind the front of the pumpkin and results in interference unless you use a bent draglink or move the steering box forward or swap the steering box for a forward pitman arm design. Moving the stock steering box forward far enough for clearance means cutting out the stock front crossmember.
This is more complicated than front shackle, waggy springs and soa but IMHO will result in better handling at high speed.
Ed Mc can give you great guidance on swapping in a 3.4gm crate motor. You have to use the trans from a Trooper or Rodeo that came with the GM V6 (2.8 or 3.1). I don't like the 4L30 autos so I'm staying with the 5 speed. I picked up a 91 Rodeo with the 3.1 and 5 speed that is providing most of the pieces needed to do the 3.4 crate motor swap. This is the simplest engine upgrade I have seen, pretty much a bolt in, plug and play, well almost. You could also use a 700R trans and Dana 300 transfer case if you want a strong, non electronic auto trans with the GM V6. To me this is an interesting option but outside the budget for my build.
All in all the Isuzu sas' are a lot more complicated than Toyotas. There is a fair amount of info out there on Trooper/Rodeo sas', less for Amigos but all my measurements suggest frame dimensions are almost identical for all Isuzus of similar vintage. IMHO the results will be worth the effort if you want something other than a red Heep. Just be prepared to spend alot more money and time than you first think it will take.
Shawn
