Alright, let me clarify some things about modular blocks.
First off, the teskid blocks were used in 93-99 (and in rare cases 01) 4V's. Starting with the 01 ford explorer and mustang cobra, continuing into the 03/04 mach 1's was a different aluminum block called the WAP block. WAP stands for windsor aluminum plant. The WAP blocks are significantly different than the previous teskids (ie. amount of material in the pan rail). They also stopped using jack screws in the mains (allows for preload, and less crank flex).
The teskid blocks are VERY strong, much more so than the iron blocks. The iron blocks are very strong as well. The WAP blocks are decent, however have been proven to need filling much beyond 700-800hp, as they become unreliable. Ford used iron blocks in the cobra's due to the reliability issue with the WAP blocks, and not being able to test them thouroughly, as well as to save costs (same block used in the GT's).
Aluminum is a much clearer choice, as it dissipates heat more rapidly than iron. A teskid block is the best overall choice for a block for any modular buildup. They weight 90lbs less than an iron block, unmodified can withstand well over 1000 bhp, and can be found for a very reasonable price of ~$300. | 
