ive done pretty extensive research on this, as i too want a 5.0. for a stroker kit by itself (ie: you are using your original block or have another block already) sean hyland motorsports has some that are very nice, strong, and reasonable. They have kits from 5.0 to 5.2 if you are willing to sleeve the cylinders. (which i wouldnt do)
a better option if you are NOT going to use your oem block is to just purchase a forged stroker shortblock from MMR. (Modular Mustang Racing) I say this for several reasons:
1. Professionally built and tested. (these guys are all over these modular motors and are making awesome HP with them.)
2. Rated for a certain HP, so you know what your limits are.
3. More economical. At 4k for the aluminium 5.0 850 HP version, i doubt very seriously that you can find an aluminum block, find a 5.0 stroker kit with that HP rating and all the needed essentials, pay for the machine work and assembly, and beat that 4K price.
4. Less down time on your car. You build the motor at your pace, then swap it over the weekend. Then you have your original as a spare!!
I plan on purchasing a shortblock from them, then finding some OEM heads and reworking them myself, then building and wrapping up the top end with a nice s/c for a bow, and installing over a weekend.
its all pricey, but this seems to be the best option.