For the T3 Chrysler or Conquest turbos,there exhaust/turbine housings & turbine wheels will be really small for a 250 CI engine (chokes the exhaust alot)
Post a pic of your T72 turbo & specs when you get a chance.
For a plenum area, really not necessary,& really have to calculate for 200 HP,but there are formulas available.
Really there is not just one correct formula that I know of to calculate plenum volume.
But here is a good design & info for a Supra
http://www.fabworks.com/assets/pdf/Supra_Plenum.pdfMBHD
