[Gimp-user] How to optimize this workflow

[Helmut Jarausch <jarausch skynet be>]

I have a dumb photo lab (which offers very good prints with a new Canon  
printer).
They offer prints of size 7in x 5in.
I have several images which are quadratic in size. Now I have to "fill"  
my image with a (say) grey area left to it.

Currently I do

I create a new image (filled with grey 868686) with the same height but  
a  width of height x 7 / 5
I copy paste my image into this new image, move the floating selection  
exactly to the left (which is quite tedious to do exactly)

Additionally I'd like to select the remaining grey area at the right to  
eventually fill it by inpainting (GMIC).
(Select by color doesn't always work)

Many thanks for some hints,
Helmut