Ok if yours is an SJ40, it originally had a 1litre motor in it. There are a couple of "types" of SJ40's, having varying levels of 1.3L equipment (disk brakes, plastic dash, wide trasmission tunnel flaring).
I have never checked it myself, But I believe an SJ70 body will fit, or at least be very close. The body mounts may vary slightly?
Its definitely been done by people before though. I would guess the hardest part would be fitting the wiring loom to the more modern widetrack dash (SJ70 is the widetrack Sierra model).
I'm not sure how to really "school" you on Sierras, they are reasonably simple 4wds. I assume you are coming from Hilux's? Sierras have a coupe of key mechanical differences from Toyotas.
The best difference to a Hilux is weight and size. everything can be just picked up and walked around with. Hilux bits weigh a tonne or 6!
The biggest thing people seem to fail to comprehend is the Sierra transfer case, it is divorced (not attached to the gearbox), and also it has a reduction ratio in high range. This means if you buy aftermarket reduction gears for the transfer they will lower your high range also. Which also brings me to saying depending on your model SJ40, you might not be able to fit aftermarket gears to your transfer (only the model with a 4wd light on the dash can accept the aftermarket gears).
Also SJ40's run LJ Suzuki size diff centres. These are smaller than the 1.3 Sierra centres, and you cannot interchange diff gears betweent the two types. You can also not get air lockers for your model, however you can get lockrights. SJ50 diffs (entire housings) do run 1.3 size centres and can be essentially bolted straight in to an SJ40 though. Your diff ratios will be 4.1, the only other 2 ratios you can get come from LJ80's (4.5), and LJ50's (4.8). 1.3 diff sizes have 3.9 (SJ50), or 3.7(SJ70), though lower ratio Vitara crown and pinions can be swapped in (4.3, 4.6, 4.8, 5.12....I think).
OK thats a bit ad-hoc
Do some reading around the place, and mess around with the car, you'll get to know it pretty quick
Wheeling on completely wicked angles, without even looking stable.
