Common problem with early LT1. Check the passenger side front bolt too ( pull the alternator ), it's probably sheared off too. The early TL1 seems to have problems with the exhaust bolts bottoming out ( possible the holes weren't deep enough when driller/tapped, and it's always the same bolts, driver rear/pass front ( same holes during machining )). This allows the manifold to warp over time with the twisting of the engine and over time the bolts shear
Did you pull the exhaust manifold off yet? Sometimes just the head of the bolts shear off and you can get a visegrip on them and spin them out.
If they are sheared off clean with the head, you are going to have to pull the head to remove them, there is no way to get to the bolt with the engine sitting back that far under the cowl.
There is one other option to consider if you can't get the rear bolt out..... The LT1 head has two outer bolt holes, the stock manifolds use one, but MAC shortie headers use the other. So you might have to install shortie headers if you can't get the rear bolt out!