180g Oceanic planning

[Delphinus88]

I just purchased an Oceanic 180g.   This comes with 2 HUGE overflows and 4 holes on the bottom of the tank.   My current plan is to remove the two 9x11x24 overflows, drill the back of the tank and install a coast to coast and use a Bean fail safe.   

1.  Do I need to set up 2 sets of bean meaning 6 holes plus the returns or will one set suffice?   Right now my intent is to have a mixed reef but don't want to be limited later down the road by low flow rates.   

2.  The holes on the bottom scare me.  On the forums I hear too much about cracked bulkheads.  It seems that would be catastrophic with the bottom drilled as opposed to only losing a couple of inches of volume if repair was needed.   What are the options?  Can I just take a 9x11x ½” slab of glass and silicone it on top to close it off?  

3. What are the advantages/disadvantages to internal/external overflows?  

 

Any other suggestions are also welcome.  Thanks in advance

Bevin