The most reliable Rover V8 is the 3.5L since it has a lot more material around the cylinders than its bored out cousins, the 3.9, 4.0 and 4.6. Unfortunately it is the least powerful as well. Bumping the compression to the 9.5 to 10.5 range nets some good gains. Also, you could keep the durability of the 3.5 by retaining the bore but rebuilding it with a 3.9 crank, or even a Buick 300 crank as some have done. This would be less smooth than the 3.5 though. Removing the cats, camming, headers, etc will only go so far because the Rover heads don't breathe very well. There are aftermarket heads now which really open up the possibilities for the motor. Combined with a roller cam and roller rockers, the 3.5L could be a real firebreather for its size, but then your talking big money. If you're after power the easiest and most effective swap would be to a larger Rover V8, such as the 3.9 or even a 4.6 converted to non-serpentine specs. There is a big difference just going from 3.5 to 3.9 due to the extra stroke. A carb swap won't give you extra power over the FI if both are in good shape. It will also decrease fuel mileage. It will give you a computerless engine, which has its merits. The 3.5L can be hopped up but the power gains will be in the top end rather than the lower end of the curve. As far as other swaps, the sky is the limit if you can get a GM engine in there, but such swaps are always more complicated and expensive than you think they will be in the end. Also, there is a point when the rest of the drivetrain won't be able to handle the extra power. For the stock Rover auto or manual transmission, that is anything over around 280-300 ft lbs of torque. Then there are the axles and diffs which are as capable as the transmission unless you mod them.