That furnace appears to be a 120k btus unit. It has a 3 or 4 ton blower, which means its rated for 1200cfm or 1600cfm with a wet AC coil. My guess is that it is rated for 1600 cfm with 3/4 hp motor.
It appears to me that you have a hot water coil that is not sized properly. Raising the water temp will not take care of the problem if coil is undersized. When hot water coils are built they look at several factors. The fan first of all. ( at what static can the fan perform up to spec ) Total btu output, usually based on 180 degree water temp. There is big difference between coils that are designed for supply use and that are used on the return side of the fan. Available space. A hot water coil can built to fit in a 19 X 20 plenum.
I don't know how you picked this coil.
What I would have told the coil supplier is that I need a coil that will deliver about 90k btus (not sure exactly what your need is) the furnace should have put out a 90k. I have a residential furnace that has a 1600 cfm blower, My supply plenum is 19X20 and I what to operate my boiler at 180 degree at 0 degrees outside. Then that coil should do exactly what the propane furnace did provided the boiler is big enough.
I would double check and make sure you have a coil that will work. Having that big of an air flow drop when the coil is placed on the furnace would make me nervous.
Let me know how you make out.
PS I can give you names of a couple of coil rep if needed.