Digital BW, The Print

Being new to this list and digital printing, this might be a basic
question, so I apologize in advance if it is.

I am using a quad tone B&W printing system and having my B&W negatives
drum scanned. For the purpose of this question, I ask for the scan to be
for output at (X) dpi @ (X) size, which, for examples sake, gives me a
75 meg RGB file. When I open the file in PhotoShop and convert it to
gray scale, I end up with a 25 meg file, which now limits me to a
smaller size/resolution print.

So, I paid for a 75 meg scan but I can only use 25 megs of it. I could
pay for a 225 meg scan to get the 75 megs I need to use or I could find
a lab that would do custom gray scale drum scans, but I had another idea
I wanted to run by the group. Would it be possible to open the B&W image
as a RGB file and then convert/save each of the three color layers as a
separate gray scale layer of 25 megs each and somehow merge them
together to end up with a 75 meg gray scale file?

Thanks in advance,

Mark Maio