455 Service Replacement Block Question

I am attempting to put together the pieces of a 1972 455 HO engine.  I have located a 1972 455 SR block that is represented by the seller as being an HO block.  The stamping number is 485428.  From what I can find, this block stamping is correct for a 1972 to 1974 455 service replacement, but I can find no reference for a SR block being specifically cast as an HO block.  I would appreciate a little guidance on this.

Thanks,

Bob  

What is the code on the front of the block on the passenger side right below were the head goes?  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

I'll check with the seller and report back.  Should it be either "YB" or "WM" just like it would be with a a non-SR block?  Let me ask an additional question while I am at it.  I have located several sets of 1972 HO, round port heads floating around.  What type of compression ratio would result with those heads and a 1972 HO block?  I know that aftermarket heads would be better, but despite the fact I am upgrading a clone I am shooting for a level of originality (and 400+HP).

I appreciate your help Pontiacman8 as I get up to speed on this new hobby.  I have a shop full of woodworking tools that I think are going to start feeling awfully neglected.

Regards,

Bob  

You would be looking at around 8.2-8.5:1CR with those heads 7F6 and flat top pistons.
No problem.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

So at 8.2-8.5CR will I be able to hit +400HP using a fairly streetable cam or am I asking for the impossible due to the limitations of trying to stay "original" with an engine that was factory rated at 300HP?  

IT should be possible with a good port job done on the heads you should be close but I would try to get the CR up to 9-9.5:1CR  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

I assume the best way to do that is by having the heads milled?  Is that correct?

Also, I just talked to the seller of one of the two purported 455 HO service replacement blocks I have located.  He seemed extremely knowledgeble and is a Pontiac racer in Mechanicsville, VA.  According to him, the 455 SR blocks did not have a block code stamped other than SR, and are only distinguished by having 4 bolt mains and nodular crank.  I have emailed the other seller and it will be interesting to see what he says.  This is like learning to cut dovetail drawers again.....

The Mechanicsville, VA guy has the mother lode though for my project.  He has in addition to the block, '72 7F6 heads,  '72 HO intake with crossover and a original '72 RA hood pan.  He also has a '72 unitized ignition, but I need to read up on that option.

Thanks Again,

Bob  

Bob you could not shave that much off of the 7F6 heads to get a 9-9.5:1CR so if you are wanting the higher Cr you will need dome pistons but even at 8.2:1CR you should be able to get a decent amount of power.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races