Exhaust Manifold Replacement

[rennalds]

My son has a 71 Cheyenne with a 350 and we are gettin ready to replace the Drivers side exhaust manifold. We have been hitting the bolts with penetrating oil all week. Problem is that one of the bolts appears to already be loose. It turns freely and wiggles but it doesnt come all the way out. Hoping this isn't a broken bolt? Also I know from other posts that the bolts are 3/8 & 16 but does anyone know how long the originals are?? If the bolts is broken and we have to drill and tap it anyone know what the right size drill bit is to accomplish this? How dow we know how far to drill in with out going to far??? Thanks in advance for your help!!!!!