Here's the instructions from ages ago:
1991 Isuzu Impulse RS Rear Strut Conversion
For this conversion you will need the original rear struts from the RS and new front strut cartridges for an '88 MR2. Different struts are available. Anywhere from Koni full adjustables, Tokico, KYB to cheap OEM type. I used Koni adjustables.
1) Cut the original strut housing right below the top weld mark. Cut all the way around the strut and remove all the internals.
2) Use a 1 1/8" hole saw and cut a hole centered in the bottom of the strut housing. The new cartridge will sit on the lip that is left on the bottom. Make sure the cartridge is centered on the lip.
3) A new threaded tube is needed. Inside diameter is 1 25/32". It's
available at Carrera among other places. Also get the corresponding top nut.
4) Ream out the hole in the top nut so that it will bottom out on the outside cartridge housing (not the inside lip near the shaft).
5) Slide the new tube over the original strut housing. Make sure the length is proper before welding the tube to the original housing. The top nut should not bottom out completely to the tube with the cartridge installed. There should be a gap of about 1/8" between the nut and the tube. Weld the tube to the original housing.
6) Install the new cartridge and tighten the nut.
7) Install the plastic washer that comes with the cartridge above the
top nut.
8) Cut the largest ring off the bump stop and install what remains on
top of the plastic washer.
9) Use a 19/32" drill bit to enlarge the hole in the top spring retainer. This hole will need to be elongated to fit over the corresponding section on the cartridge. A dremel tool works well. Be careful not to oversize this hole. It must not be able to slide all the way down the shaft. It needs to be very snug. Reinstall the top spring retainer.
10) Using the same 19/32" bit, enlarge the hole in the top mount. Reinstall the top mount.
11) Use washers or appropriate spacers next so that the nut holds down
the top mount. I had to use 5 washers.
12) Use the same 1 1/8" hole saw to drill a hole in the seat belt mount
bracket just above the top strut mount in the car. This will provide access if you're using an adjustable strut.
13) Reinstall the strut.