Line up #1 Piston at TDC REGARDLESS of crank pulley position.(verify with removed head) THEN, set up cam pulleys to the timing marks. as long as the piston is at TDC, If you set up the cam pulleys to the marks, the engine will run.
Working with SBC's, I usually verify the piston is at TDC visually, before lining up cam marks. I've seen timing chains have the marks machined wrong, and setting them up to those marks opens a world of problems.
REMEMBER...TDC happens 2X EVERY cam rotation. TDC on Compression, and TDC on exhaust. IF you set up TDC on the exhaust stroke, THEN turn cams 1/2 turn,WITHOUT turning Crank, you will be at TDC on Compression.
as for how your problem happened, I have no clue..