Q&A

 Master

Double check that you have correctly reconnected all the vacuum hoses and electrical connections. After that get a friend to crank over the car and with a squirt can with engine oil in it squirt oil around the intake manifold to cylinder head surface see if the oil gets sucked in at any point which would indicate the intake manifold gasket has some how got pinched or installed wrong causing a lean condition.

Likely yes, it is your timing. You need to put the distributor back into *exactly* the same gear tooth from which it came. If you don't, you're either going to have rough idle or no start at all.

If you bungled that procedure (i.e., you didn't match-mark the distributor body to the intake manifold) there's no way you can adjust the distributor location to make up for the mistake (which of course you could do by rotating the distributor body in an older car). Instead, you'll need to reset the distributor by getting the engine at #1 TDC and placing the distributor in per GMC's recommendations.