Firstly it probably wouldnt fit. But more importantly the lorry's engine has a max rpm of 2000. The reason for this is that lorry engine's are designed to produce vast amounts of torque not just BHP. Thus torque means turning moments. This means that torque is the amount of turning moment the engine can do to the wheels. So lorry engines are designed to be able to pull massive loads. Therefore if you put a lorry engine in your car firstly it would weight it down and cause massive amounts of understeer. Secondly your car would now accelerate slower than it did before but would be able to accelerate massive weights assuming your car's framework held together. If you put an F1 car's engine in a lorry it would not be able to move depsite having 700 more BHP than the lorry's engine. This is because the F1 engine cannot create enough torque to turn the wheels and would stall indefinetely.
(N.B. This is a simplification, F1 engines may indeed produce enough torque thorugh different gearing)
You are also wondering why a lorry now needs so many gears. Even though it has a range of 2000rpm. To keep the engine in its productive rpm range it needs loads of gears. Therefore if you have, like me ever driven a lorry you notice how in first gear you stop accelerating very quickly. And if u dont change it reaches a max speed very quickly in that gear (roughly about 5mph)
So in summary torque is the ability to turn the wheel of a car or lorry. If an engine has lots of torque it can pull large loads. So low end torque in a car would mean that in the low rpms of a car it still can accelerate e.g. Evo VI over the Subaru P1.
Both cars have roughly same BHP but the EVO VI has more torque so it can still accelerate better than the P1.
So if you have two cars with the same weight. The car with more BHP will acclerate faster. the one with more torque will be able to pull a larger load. Because torque and BHP are linked it is very likely that the car with more BHP will have more torque.