Some more information about blocks and interchangeability:
Mustangs have come with both Romeo and Windsor engines. The 1996 thru 1998 Mustangs came with Romeo based engines. The 1999 thru 2000 Mustangs came with Windsor engines. Then just for fun, Ford went back to Romeo engines in the 2001 thru 2004 Mustangs. Be sure to check the engine no matter what year it is. It seems that Ford carried over some engines into the following model year.
4.6L DOHC engines have come with blocks cast in both iron and aluminum. You can find the aluminum blocks in Mustang Cobra's up to year 2001, Lincoln Continental, Lincoln Mark Series and the Lincoln Aviator. These blocks are cast by Teksid in Italy for Ford. All of the aluminum engines are assembled at the Romeo plant. No SOHC engines have ever been cast in aluminum, although the blocks are generally interchangeable between the DOHC and SOHC. The only exception to this is the Continental engine which is front wheel drive. This block will not work in rear wheel drive vehicles. One of the motor mounts is not drilled, the bell housing is different to allow mating up to the transaxel, the oil filter is located on the opposite side of the block, the front cover and accessories are different. The oil pan has a front sump so this may help with some swaps that require a front sump. If you get stuck with one of these engines, you can still use all of the internal parts, cylinder heads and intake. These parts will bolt up to an iron or aluminum rear wheel drive block. Be sure to evaluate the time and expense associated with this kind of swap, all of the necessary parts needed to complete this conversion will add up quick.
The Lincoln Mark series are probably the most popular 4.6L DOHC engines, due to the relatively low
price compared to the Cobra engines. What are the differences between these two engine's? Following
are some, but not all of the differences.
The Cobra comes with a forged steel crank using 8 flywheel bolts, the Lincoln comes with a nodular cast iron crank with 6 flywheel bolts.
The Lincoln uses a different oil pan with a reservoir on the side.
The oil filter/water outlet castings are different.
The Intake cams on the Lincoln are a slightly different profile.
The Intake on the Lincoln is different from the one used on the Cobra, although they do use the same cylinder heads and a Cobra intake will bolt on.
The 6.8L V-10 has never been cast in aluminum except for a limited number that Ford did for a Mustang concept vehicle. The 5.4L DOHC has been cast aluminum but has, up to now, only been available in the supercharged GT40. We will have to wait and see if Ford offers this block through it's performance parts division. Just a guess, but you probably won't find many of these at your local pick-a-part yards.
4.6L SOHC - 500 lbs.
5.4L SOHC - 525 lbs
4.6L DOHC Aluminum Block - 425 lbs
5.4L DOHC - 540 lbs
6.8L SOHC V-10 -640 lbs
Most people that are doing these 'get in and get out' comparisons are used to a geared 5.0L. I know on the peak numbers, the 4.6L makes the same torque 100 rpm higher...than the 5.0L. 
