Actually the Rover V8 is very light. Especially when compared to a TR6 engine. Remember that both the Block and Heads are aluminum on the Rover V8. The original displacement, as in my TR8, is 3.5 liters (215ci) and was originally a pretty low compression engine for emissions reasons. The horsepower was not really high but the torque was and the torque curve is pretty wide.
Restricted breathing was the stock Rover V8's downfall. The carbs are small and the exhaust pretty restrictive. Opening up the intake and exhaust can really wake up a stock 3.5 V8. By far the biggest power improver though is different heads. The original design was very restrictive but later engines had better flowing heads. If 3.5 is too small, you can also get the engine in 3.9, 4.0, 4.2, 4.6, 5.0 and have even seen some go to 5.5 liters.
A small block Chevy or Ford will not save you any weight over a Rover V8 but inexpensive parts would definitely be in their favor, especially for the Chevy.
The Rover engine is also fairly wide. A small block Ford is actually a more compact unit. If you spring for an aluminum block for the small block Ford you would save another 60lbs and could edge ahead of the Rover V8 for a lightweight V8.
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