When you say 3.9 Range Rover gearbox, what do you mean?
It is possible to fit an SBC, however you need to decide if you want to:
-fit it to your current gearbox (or another Land Rover box), thus retaining the standard transfer box
-use a GM gearbox that easily mates to the engine and fit it to a Land Rover transfer box
-use a GM gearbox and transfer box
There are pro's and con's to all options. And on a project like this, you'll need to research and understand these differences, so that you can make informed decisions on which route to go.
Now, as for the engine, in the UK we usually get totally confused and just call almost any American V8 a 350 Chevy or small block.
In reality the 350 Chevy hasn't been a production engine for almost 20 years now. Although you can still buy new crate versions of them. The thing to note is, they have a cast iron block and vary wildly in power out, from as low as around the 120/140hp mark upto the highest production version of 330bhp as the LT4 in the late model C4 Corvette.
A low powered one will need new heads and a cam and lots of other bits to make good modern power as a rule. Although they are very tuneable. But will depend which variant you go for.
However.....
In 1997 GM launched a new engine known as the LS1, it's still 5.7 litres, but it isn't a 350 Chevy, although you can still call it an SBC. These are 100% completely new design and share no parts with the older engines, they also have alloy blocks and are lighter and much more powerful. But they are also more expensive to buy.
http://www.marks4wd.com/ are in Oz, but offer quite a number of different adapters that you might find of interest, although none are cheap.
Now something else to consider.
Stock axles on a 90 are weak, you can break them with a 2.5TD easily. And Rover gearboxes are not the strongest either. So you are likely going to want to consider and at least research some upgrades to these, or even alternative axles if you plan to off road. And if you are on a drum rear axle, you'll want discs and likely vented up front.
I like V8 engines and American ones, but even if you do a lot of the work yourself, of which there will be a lot, as in a lot of custom and bespoke bits. This is still going to cost a lot of money. If you are paying someone to do it, then it'll be even more.
Honestly, a good 3.9 or 4.6, maybe with a cam and a good exhaust and fuelling setup is going to be more than enough in a manual 90 and will pretty much fly.
If you want oddball, the 2.0MPi out of the Disco commercial will fit a 90. And it would be comparatively easy to substitute in the 2.0 Turbo variant from the Rover 620/820 Turbo. 200hp straight off, easy tuning, although you'll likely want to upgrade the pistons, but 300hp is perfectly doable from such a setup and likely at the fraction of the cost of a Chevy V8 conversion. A good 3.9/4.6 RV8 should get you to a fairly healthy 230-250hp without major cost, vs the SBC route.
( kicks himself hard, again and again and... 
