You could possibly get a cyclone in there. The 4.6L V8 does fit in there too, but its best to pull that from a Continental. I would shy away from large displacement however. If I were looking to put out lots of power, I would go the forced induction route. Probably start with a SHO motor or a modified Duratec. Then build the engine to take boost, then build the transmission to handle that kind of power. That way you can use a reprogrammed stock computer to run it all. Otherwise you need a manual trans, which isn't a bad thing, and a stand alone fuel management system, or you need a standalone transmission computer and a standalone fuel management.
But with time and money and fabrication skills, you can fit just about anything into anything. Personally, I wouldn't want to run an antiquated pushrod V8 in a Taurus when I could produce more power with a turbocharged V6, and it would be lighter, so the handling would be better. Unless you are talking about the new 5.0L V8 that just came out.
Yeah, realistically you are best sticking with something that would actually be found in a Taurus, Lincoln Continental, or Jaguar X-type. You can also use the Contour and Cougar engines, but those are for the most part considered downgrades to the 3.0L Duratec.
Yeah, the name Duratec has been used loosely with a number of Ford DOHC engines that are not necessarily in the same family. You probably could fit that motor into a Taurus. Still, I lean towards turbos.
Yep. I know thats not exactly easy to do, you can twin turbo a Vulcan too. The key there is firstly, upgrade the motor to handle the boost (the stock motor can only tolerate light boost). You don't just want a setup that will last a short while or give up the first time something goes wrong, so building the motor is the ideal starting point. There are companies that sell the forged pistons and stronger rods. You might be able to get a stronger crank as well. The ports should be enlarged to allow more flow , and it wouldn't hurt to use larger yet lighter valves. You will also probably want to switch from using head bolts to head studs. They will hold the head tighter to prevent blown head gaskets. No special gasket will be needed unless you want to run a thicker copper gasket to lower compression.
Then you get larger injectors. There are outfits that could recommend the right injectors based upon your displacement and the target horsepower.
Custom manifolds will need to be fabricated. There are small turbos you could use, I've seen some pretty small units on 3000GTs that produced some pretty serious boost. Even though the engine compartment is tight, these would fit quite well, probably better than cramming a huge motor in there.
Routing the intake and intercooler lines will be tricky but must be kept in mind The Taurus does have plenty of room for an intercooler behind the bumper. If you run this correctly, you can maintain the coolant reservior and such in the proper locations. It is recommend to relocate the battery. The air intake must draw its air from outside the engine compartment. You will also probably want a better radiator than stock.
The transmission will need to be either swapped to a manual trans or upgraded to handle the power. The AX4N will handle insane amounts of power if properly upgraded. You will want to get both the Transgo and Sonnax shift kits and additionally, you will want upgraded bands and clutches (Red Eagle is the best brand), and a much sturdier torque converter. The manual trans will be easier to deal with.
For fuel management, the stock computer can be reprogrammed to handle a boosted engine. There are aftermarket computers as well, they work even better but few if any can control an automatic, meaning you either have to use a manual (recommended anyway), or you need a standalone transmission controller. Either route works well.
Achieving 300 hp is not hard when dealing with boost. With between 7 to 12 lbs boost you can meet or exceed that level. Since you are going to be lighter than with a bigger motor, the car will be quicker too.