$370 sounds decent for rear main replacement. The seal is really only about $10 and takes maybe 5 minutes to install, but it's a PITA to get to. Some would drop the trans, others would pull the engine.

Dropping the transmission (about six hours your first time):
- Lift the car and remove the front wheels.
- Drain the transmission fluid. (2.1qts capacity)
- Remove lower bolt in strut fork.
- Unbolt lower control arm where it mounts unibody.
- Leave bastard nut on the outer end of the CV and use brake rotor as handle to pull/pop the CV out of the diff. (Or use a pry bar to coerce it from the diff.
- Free the clutch slave from the front of the bell housing.
- Remove your battery, battery tray, intake pipe.
- Disconnect the shifter cables on the top of the trans.
- Support the trans from below with a jack (use a piece of wood to protect the cast aluminum bellhousing).
- Take the block-to-bellhousing bolts loose.
- Take the (2) trans mount center bolts loose.
- Size up the situation and take care with separation of the trans from the block. You don't want to just have it fall on the floor. Cast aluminum is a bitch to weld back up.

Since you're going to go through all that trouble, you ought to consider a new clutch and TOB while you're at it. It's even easier to replace than a headlight bulb once you're in there. Give SBC a call and see what they can do for you in terms of a nice street clutch.

Have fun.