yeah, I would agree, with the work it would take to swap it piece by piece, you might as well swap the entire sub frame as one piece.

BUT, you never know when it comes to subframes, you might find that the one on the shell is in better shape and yours is rusted or something dumb like that. So inspect thoroughly before tackling that project.

bushings - (don't know if price is good, or if the site is good, just pulled it up with a quick search, but it is basically what you're looking for)
nismo set - MyNismo.com - NISMO Bushing - Rear Subframe Insulator Mounting
energy suspension - MyNismo.com - ENERGY SUSPENSION - Subframe Insert Set
energy suspensino - MyNismo.com - ENERGY SUSPENSION Rear Sub Frame Set

From what I remember, the nismo ones are supposed to just be harder rubber. Much improved over stock, but still not as stiff as the energy suspension, polyutethane ones. If that is true ( I honestly don't remember) then the nismo ones might be worth considering for a daily driver.
I don't have a personal opinion. I've driven either stock, or aluminum ones (or my buddy's welded together one)

might consider cleaning it up and painting it to ward off any rust issues down the line.