350 is realistic if:
5.0 big bore and sleeved
10.5:1 compression with forged internals
FR500 high lift valvetrain and cams
Worked Terminator heads
FR500 or equivelent, like a worked Aviator intake.
Teksid block is the best (F2LE casting). The Teksid is the heaviest aluminum modular plant weighing 86lbs. It is also the strongest because of a 6-bolt main and web reinforcements. It is alleged to handle up to 2000hp with proper modifications and preparation. This should be the block you choose. The WAP (Windsor Aluminum plant) sucks- that's what you find in 00-02 Cobra's and Marauders, Mach's, etc. It sucks, avoid it. Besides, a Teksid block is going for $300-500 in excellent condition right now. You can even buy the new from Kar Kraft.
As for long block vs. short; buy shortblock, so you can have more of a choice in heads. Go with Terminators (03 Cobra, Marauder, Mach I) but make sure that the heads are either new or have a blue or yellow spray paint marking; those are the improved 4V head that has crunched the old valve-ticking problem. The problem was caused by a casting flaw that interfered with the cooling of the 7 and 8 cylinders. So if you see some bozo selling "cheap" used 03 Cobra heads- do a little research and trust no one; one Corraler found that out the $1500 hard way.
See VT Engines shortblocks...