Add string to boolean calculation

I have written the calculation below. It get's me 90% of the way to where I need to be, but without the other 10% it's worthless. Hoping someone can help me with this one:

IF sum([header sum])>=1000000 then sum([header sum])/1000000

ELSEIF sum([header sum])>=1000 AND sum([header sum])<999999 then sum([header sum])/1000

ELSE IF sum([header sum])<1000 then sum([header sum])

END

END

The calc evaluates the 'header sum' value and converts it to trillions, billions, or millions as appropriate. I need to tack on the appropriate text (in red):

IF sum([header sum])>=1000000 then sum([header sum])/1000000 + 'trillion'

ELSEIF sum([header sum])>=1000 AND sum([header sum])<999999 then sum([header sum])/1000 + 'billion'

ELSE IF sum([header sum])<1000 then sum([header sum]) + 'million'

I suspect the answer may involve wrapping things in STR(), but I haven't been able to make it work.

Apologies for not posting a packaged workbook. It's proprietary and trying to scrub it was taking forever.