I don't know if this is going to help or not, but I found this reference for head flow numbers for a 320 straight 8. I don't know enough about the straight 8s to say whether these flow numbers are comparable to your 263.
http://users.erols.com/srweiss/tablehdc.htm#Buick
Are the head flow numbers measured from an actual 263 head or are they the generic files built into DynoSim5? How extensive is this build going to be? Are you planning to have the head ported? Custom pistons for improved compression ratio? Maybe some custom built headers?