As for the 3.5L V10's, they were used up until 1997 I do believe, and were switched to 3.0L units for the 1998 season along with the introduction of the narrower track cars (1.5m down from 2m) and grooved racing tires, this was all in an attempt by the FIA to reduce the speeds of the cars, which really didn't work too well in the long run
As for the cars with the 1.5L turbos, again, that was a different era, lol. Back in the 80's, teams could either use a turbocharged 1.5L engine (I4 or V6) or a naturally aspirated engine of 3.5L displacement (V8, V10, or V12). So that is why some cars in GT3 were 1.5L, and some were 3.5L. Well... actually the F090/S and F094/S/H had no choice, because as of 1990, the FIA banned turbocharging, the reason Honda left Formula One actually, until they came back all out in 2000.
As for the comparison, yes, you're pretty much correct

CART still uses a "manual" gearbox, in that it is a sequential shifter (pull towards you to shift up, push away to shift down) whereas in F1 they are now fully automatic (banned next year) and they have paddles on the steering wheel to shift (left to shift down, right to shift up), and a fully automatic clutch. As of next year, the drivers will have to engage the shifting themselves (using the paddles) and will no longer be able to have the ECU do it for them (Ferrari's system anyway can shift a gear in LESS than 5 one-thousandths of second

), however the clutch will still be automatic as always, it's just they will have to have a third mini-paddle on the wheel for it at the start of races, as launch control is to be banned as well. However, I'm not so certain these devices will still be banned, as the FIA came to some agreement that if the engine manufacturers (Ferrari, BMW, Mercedes-Benz, Honda, Renault, Toyota, Ford) can supply customer engines at an "affordable price" of $10 million per season, these devices will not be banned, however I'm not sure how that is playing out anymore.
Again, I hope I helped
