The reason I ask is -I already have the A5000 (which I'll probrobly be stuck with, due to popularity of twins.) and I saw a good deal on the A3000. So, I figured no harm in asking.
You could find a bigger turbine housing for a k31 off an older 60 series Detroit.
I wouldn't think the 5k would be horribly undersized for a bottom charger. I haven't ever see a compressor map for a 5k but they seem to move a fair bit of air.