Help! Cranks but won't start

[Lated]

I took my truck out for a drive and didn't make it far before I ended up on the side of the road waiting for a tow truck. Truck had been running fine but while going for the 1-2 shift, I heard a pop from under the hood and the engine died. I tried cranking the engine but nothing happened, the lights wouldn't come one, nothing. After popping the hood I noticed my 50 amp main fuse was blown (installed when I rewired the truck a few years ago with a Painless harness). I wired around the fuse just trying to get the truck home and now it cranks but won't actually start.

Where should I start looking for problems, and any ideas what could have pulled enough juice to blow the fuse in the first place? Truck has a small-block 400 with unknown internals and an HEI dizzy.