I *have* seen a Firenza with a big block Chevy engine in it, but I'm glad to say I never saw it attempt to go round a bend
Having got a Ford V6 in my own Viva and knowing how unbalanced this makes the car I certainly wouldn't advise adding another 150lb
with an SBC, as nice as the prospect of 5.7 litres might sound. The lightweight Rover V8 is unquestionably a better upgrade for a
Viva as it only weighs the same as the stock 1800 lump. Rover engine can easily be tweaked up to 250BHP which is more than enough
for such a lightweight car.
Arguably you might get away with the SBC if you were to fit a Jag IRS at the rear to balance the weight better and give more axle
control.
I had the Jag IRS on my old V6 HC and that actually handled a *lot* better than a standard Viva, though 205s were essential on the
front to stop understeer, whereas my HB with the same engine but a stock magnum axle, and only room for 185s up front, understeers
badly and is very twitchy when pushed hard as the rear axle twists all over the place.
Sid