DIYbanter - Constructing air returns...

Can someone please give me or point me to a decent description of what
construction methods are allowed for air returns in a residential
application? If you're going to refer me to the code, please cite
volume, chapter and verse for me...

Specifically, are we still allowed to box in the space between a
couple of floor joists as part of a return air run? If not, what else
*can* be done?

Details if you want them:

The house is a '50's vintage wood frame bungalow in southern Michigan.
The original gravity furnace and supply ductwork were replaced with a
gas forced air system. The original returns were left in place and
look like they will be adequate.

We're planning to finish our attic into two rooms. Each gets a supply
and return. The RA grilles go on a hip wall, and are ducted between
joists to the middle of the floor and drop through part of a closet
for one of the rooms, and two stud bays in an interior wall for the
other.

These stacks will be made from galvanized, like everything else in the
system, and safely enclosed. Happily, they'll come out right on top of
the main return air duct in the middle of the basement ceiling so I
can connect them up directly.

It's the horizontal run in the attic floor that I'm not sure how to
build.

Before you ask, I've already spent a lot of time doing my homework for
this project. I've done the heat loss calculation and confirmed that
the furnace is well sized to the new load. I've verified the duct
sizes and airflows for the system, and in fact I'll be changing one
supply run that the original installer made undersized. The work is
going to be permitted and inspected, and I'm not going to do a thing
until I know I'm doing it safely and right.

If you can shorten my search for these particular answers I thank
you...

Mike D.