Hey
I had 2 soft spots in in front of the tramp of the same size, and fixed them while the boat was on the trailer (hopefully the 1979 boat will have a long life yet

)
what i did was move some crates to the front and read of the pontoons to add some support, rather than just resting on the trailer. Had no issues beyond that. If the soft spots are under the tramp, there were also two of the same size again (i wonder if all the repairs were worth it at ths point).., I left the pontoon on, and removed the trampoline and just worked around the structure, not too hard, just have to bend at odd angler.
I also used West system's Epoxy in about 75 degree weather. they have a slow rate hardener that should work for you.
make sure after drilling the holes, start uphill if you're using the injection method..... i found out the hard way