i know nothing about this engine except:

Block is 1969 350
Heads - 70 GP 400 350hp (13)
Edelbrock single plane
HEI
Holley 600CFM

Paid 500 cash..no option to further inspect..comp. test etc.. I took a gamable.. paid on the spot ... & after dealing with the motor ount issue - dropped in my 71 bird..the motor runs strong.. real strong.. but I just blew the rear main seal..
my question...concerning the vacuum setup - I have valve cover breather & Valley Pan PVC connected to full-time vacuum intake baseplate.
Is this correct..??  Could my vacuum set up be incorrect - to much crankcase pressure?.. or just bad luck.. only put a few hundred miles..  

Let's see if we can sort this out for you. There are three possible problems here.
First, you say there is vacuum on both the PCV and valvecover breather. Let's look at how this system works. The PCV valve in the valley pan draws oil vapors from the crankcase into the intake manifold by vacuum, to be burned instead of venting to the atmosphere. The valve cover vent should either have a tube going to the air cleaner or a push-in breather to allow clean air to flow back into the crankcase, either way the incoming air must be filtered. The way you have it setup, the crankcase is under constant high vacuum, causing gaskets and seals to operate under suction, which may weaken the seals causing a leak.
The second possibilty is a just a dried out rope seal, they tend to dry out if they sit around without running for awhile or it wasn't installed correctly in the first place.
Third and least likely (because you say it runs strong) is worn or broken piston rings. Does it smoke under accelleration or load? Do a compression or leak-down test, leak-down is more accurate, before you pull the motor to replace the seal.
I say "pull the motor" because an old dried up rope seal is nearly impossible to remove completely with the crankshaft in place. Now would be a good time to replace the seal with a 2pc neoprene seal from BOP Engineering, available from their website.
Hope this helps.  

thanks for the reply... yes i do have a breather on the valve cover & the valley pan PVC hosed to the fulltime baseplate vacuum.  Beleive the motor had some extended downtime - so the rope seal is probably shot. It does not leak - unless its running - & 'spurts' when revved.. so its the rear main..

Thanks for the help..  

I like those new 2 pc. Neoprene rear main seals. I've changed out enough of the rope seals under the car that the next time I'll just pull the motor, stick it on a stand, and do it standing up- much easier.

Geno