Wood furnace plenum design

[KM1]

Hi All, I've got a house that has 25% basement under it, and 75% crawlspace. The wood furnace is in the basement section, with ducting run in between/under floor joists in the crawlspace part.

I had a 100kbtu wood furnace that was all burnt out, so I replaced it with the same model.
In the process, I found that both the supply and return air ducts were only 8x16" (128 sq") vs the 240sq" return, 220sq" supply the furnace manufacturer wanted.

Ducting has all been replaced, a few extra heat registers run, and I've worked my way back to the furnace.
It's going to take a few awkward bends to go from where the ducts enter the basement, into the plenum.

Any advice on plenum design?
The mounting lips for the plenum are 24"x24" (I think).
Is there any advantage to having a swooping 180 degree turn out the top of the plenum, coming down into my duct, (think an upside down U) vs having a square, capped off plenum, and coming out the side with a rectangular tap in?

My gut says that a big sweeping 180 would have way better air flow, but maybe the blower will generate enough pressure for it to not matter.

Any help is greatly appreciated!

 

[homemade]

I’m no expert but I feel duct design using box plenum vs round that is twisted into shapes has little difference unless you start messing with smaller or larger then required size.