Air Conditioning - moving duct trunk line

In my home, the main duct board trunk line is on the attic floor, as is the air handler (Carrier). Above the trunk line, at the roof line, is hanging a 20" flex air return.
The trunk has a number of branches to registers, most of them metal pipes, though some are flex.

I would like to replace the trunk line and hang it up just next to the return air flex. Is this OK?
If this is OK, I would build the trunk line from duct board (as it is now), but replace the branches with flex pipes. Is this fine?

Finally, is duct board the best option for a trunk line? What other options are available in addition to metal sheet?

The AC is a 5 ton Carrier (split) with the air handler on the attic. The first 3 feet of the trunk line (starting from air handler) is 16"x18" cross section, transitioning for the next 17 feet into 16"x12". Then a 16" flex pipe joins it to second part of ductboard trunk of 29 feet. The latter is alll 16"x12" cross section.

I intend to do all this work myself. Is it doable? This would be my first encounter with work on AC.

Thank you in advance for your answers.

I received no answers to my post of 12-17-07 concerning "moving duct trunk line". If I did not include enough details, please let me know. I really would appreciate some help with that project.

Thanks,
ab

The problem in hanging duct board is support. You almost need to run a piece of plywood under the duct to prevent the hangers from cutting into the fiberglass. If you change from metal to flex, you will need to upsize the flex by one size. Flex has far more resitance to air flow than does metal.
Do you have the tools for fabricating duct board?

Thanks for your response, Grady.
It will be a lot of work, but I can deal with builing the support for the duct board. The main question is therefore is whether I should replace the duct board with duct board or with metal sheet or with flex. Which one is best for health reasons, and for working on my own.
I would not have a helper, and no, I don't have any special tools for that kind of work (not for duct board, and not for metal sheet). What do you think?

Thanks,
ab

I much prefer sheet metal for durability & health reasons. You can buy prefab sheet metal duct which only requires some simple hand tools for assembly. Be sure to seal all joints with mastic made for the purpose then insulate the exterior with foil faced fiberglass duct wrap. I have no problem with short (<10 ft.) runs of flex from the trunk to the registers. In fact, the flex will help to dampen noise from expansion/contraction of the metal duct.

Now I might be a bit biased here considering my 28 year background as a Sheet Metal Worker but I would probably rip all that crappy ductboard stuff out of there and go back with some beautiful sheet metal duct all hard piped (maybe some short flex runs) with seams sealed and 1 1/2 ductwrap insulation. size it per manual D and you will probably be good for a couple of lifetimes. Do it right Do It ONCE.

I agree with Saturn with the exception of the wrap. In my area, we have to use a minimum of R-6 (2.5") but I prefer 3" (listed @ R-8.6). No such thing as duct being insulated too well.