As I understand it, the general objection to turbo's is that the boost build
is non-progressive.. it tends to hit all at once. This plus the enormous heat
buildup of the turbo and plumbing certainly is a problem for any installation
on any motor.
While in theory, in a given case the supercharger would deliver equal stress
for equal boost, the power flow is much more linear and the heat build less.
Ergo the functional stress on the motor is moderated, even if the measurable
stress were the same. Most materials in our dimension handle gradual build-up
of stress much better than sudden shock. B series lower ends are no exception
to this.. the turbo attempts I've seen and heard of ended in a symphony of
rods playing on the underside of the hood. Of course you can tune and
wastegate your way around these problems to a certain point, but "just
because you can, doesn't mean you should."
If you prefer to use the ad-hominom argument, there are a fair number of
sucessfully supercharged A and B series motors running around, and a serious
lack of turbos. That does say something about both the stress issues and the
ease of conversion as well
As for power losses and stress to drive it, a modern small compressor
supercharger doesn't drain more than a few horse from the motor, not really
worse than what happens when folks hang a heavy duty alternator plus small AC
unit on a B.. which is done often.
The car you refer to would be a Z-type. The Y-Type was also a saloon but was
powered by the XPAG engine (same as MG TB etc). There is a Y-Type in the UK
which benefits from a Shorrock blower (owned by Frank Vaultier - not sure if
my spelling is right). Makes a 'distinctive' noise and moves quite well.
As to the strain on the bearings, don't forget that with a supercharger the
crank etc have to develop the power to drive the blower. For similar power
outputs turbos are generally kinder to engines.