Steve, my mind has been over active on the car front, unlike my motivation to get up to the garage...
However, have you seen a saab turbo dropped into a dolly? I know you have mentioned the vauxhall versions, but the saab b234 seems to be a rather good engine, Triumph family (just) but I haven't seen one fitted. But you may of one, or know why not?
The SAAB engine is run with the pistons vertically aligned and rotated 180º in a SAAB, driving the front wheels....
Alun's comment is not quite correct, the engine used in the FIRST Saab turbos (1979> IIRC) which is a development of the Triumph 1509cc (later 1854 and 1998cc) slant engine concept runs with the pistons inclined at 45 degrees to the vertical, just as a Sprint engine does. But because it's mounted in the Saab body with the flywheel forward and the timing chest hard against the bulkhead (rotated 180 degrees as Alun correctly states) the lean is towards the drivers side of the car. None of those discrepancies are really relevant to the conversion into the Sprint, except that the 45 degree tilt results, as it does in the Spint itself, with no good place for a free flowing exhaust manifold, let alone a decent sized turbocharger, near enough to the head to reduce lag to an acceptable level. Nor does the Saab's use of a belt driven water pump materially affect a conversion.
A bigger problem, is that the Saab unit is indeed FWD and uses a 1300FWD Triumph-like gearbox (but considerably more rugged) mounted, along with the diff, underneath the engine and which, obviously, contains the engine oil sump. So you'd have to fabricate an entire sump UNLESS the block design is still close enough to it's Triumph roots to accept a Sprint sump (wouldn't that be nice?) The same possibility exists for the gearbox mounting to the block. If the block is still similar enough at the rear end, it will accept the Sprint adaptor plates and gearbox. If either or both of these fittings aren't still the same, you are going to end up with a job that probably isn't worth the effort.
Whereas the later B202/4 engine DOES sit almost upright (7 degrees from vertical, if you want to be pedantic) leaving plenty of space for a sizable turbo, has a sump which aligns nicely with a Triumph subframe, mates perfectly with a Carlton/Omega/Manta gearbox through a readily available clutch and is strong enough to tune to 400bhp before expensive bottom end work is necessary.
The biggest problem I predict with the B202/4 engine is making suitable mounts for a longitudinally fitted engine when it was only ever designed to be fitted transversely. I actually had a B204 SE spec 150bhp turbo engine in my yard as I was seriously considering this as an option, that was the biggest stumbling block I could see. One of the main reasons I favour the Vauxhall engines is because they WERE used in longitudinal apps, even though the vast majority were transverse, all blocks carry the mount points for longitudinal fitting, which makes MY life easy peasy!
If you'd like to try it on, despite these possible (and definite) problems, there is a Saab specialist breaker near me, I might be able to liberate a B234 block to experiment with for not a lot of dosh!
PS, The B202 and B204 are not "Vauxhall engines" they are pure Saab. A carry-over from when Saab were an independent company. They were adapted, after the GM takeover, to accept Vauxhall gearboxes, which is handy! In fact the Saab 9-3 is on a Vectra A /Cavalier III floorpan, the only difference visible from below is the lower wishbones on the Saab are cast alloy where the Cav ones are pressed tin!