I will give you cubes rule for torque. HP is a calculated value from torque and RPM.
Yes you can reduce the weight of the big blocks with aluminium heads and intake. Lightened reciprocating components can take more off. And you are correct the same can be applied to small blocks. The effect will not be as dramatic due to the inherently smaller component sizes reduces the amount reduction. Now some engines respond with dramatic weight reduction with aluminium heads and intakes. The hemi is one. The volume of cast iron used to cast those monster heads is phenominal. Another is Ford FE series when Al heads and intake are used. Their heads are not too bad, but the Fe intake is a brute! Now look at the SBF. 289 and 302 came with cast 2V intake and weigh about 480 lbs. Replace the top end with Al and that already compact and light engine is down to close to 400 lbs.
The bottom line is intended use and expectation. If you want to tow a loaded 5th wheel trailer daily, a big block has the meat to handle the stresses and vibration better. Can a small block be built to the needed torque and power requirements? Usually yes. But it will normally not last as long. That is why heavy duty truck engines weigh 2500 to 3000 lbs. Lots of cast to absorb the generated stresses. Then there is the industrial engines that weigh tons, run at 1300 RPM for months on end. It is intended use.
The big block in city driving can tend to negatively effect fuel economy. But you put tall gears in and let it idle up the freeway and fuel economy will be fairly good. Take that same vehicle and be a boy racer with 4.30 gears and you best be partners in an oil company.
I stand by my statement, a regular street driven car or light truck is better served with a small block. Drag racing or towing is better served with a big block. In the '60's many people with holiday trailers owned cars with big blocks to tow them. GVW was not monitored as much as the people driving Caddies, Olds, Buicks, Pontiacs, Chryslers and Lincolns did not seem as inclined to rip up freeways at 90 MPH while towing a fairly large trailer. Now you have the village idiot crowd that can not read functionally, driving a lifted 3/4 ton pulling a trailer at 90 MPH.
