I had the same problem with my mobile home and changed from a split system to a gas heating, electric cooling package unit. I ran metal round duct underneath with external insulation all the way to the center of the home and tapped into the existing distribution system about mid way. I ran a new return air underneath also and placed a new return and metal grill can in the corner of my living room. I put the minimum of 4 self tapping 3/8” screws equally spaced around the duct joint circumferences and sealed all the joints including the snap lock joint (long duct joint).
Originally when we purchased the home 30 years ago (new) they wanted to sell me a 2 ton and I told them no I want a 2.5 ton. Having dealt with rule of thumb contractors selling and installing mobile home units in Ft Lauderdale 40 years ago I knew some of the trade tricks. I still get a possum or two in the last 7 years or so but I put out food in the back at night and shoot them when they come out. They have not torn up my insulation on the metal duct yet.
My mobile home is about 16x75 and the 2.5 tons works great even when the outside temp was 103 a few weeks ago. When I sized the duct I oversized (liberal sizing) it knowing that the existing distribution system was undersized which seems to be the standard anymore. The chart you sent is similar to what I mentioned. Check and see if the speed tap is set on Hi for either the 230 or 208 volt as applicable. On the 230v, it appears that you can only get 1467 CFM if your static losses are no greater than .45 inches which should include duct losses, filter loss, and grill loss. So you probably would not get 1467 CFM but something less. Liberal sizing for the FURNACE trunk duct at 1400 CFM would result in an 18" dia duct.
Note that if your new unit evaporator coil (indoor coil) is freezing (everything else being correct) than you fan is either not working, you have a blockage in the duct or just not getting enough air a crossed the coil due to increased duct etc. losses.
Is your roof cool sealed (light reflective mopped on sealant)? Do you have an exorbitant amount of glass is the glass you have double pane insulated?