You should have enough men to do it. In the opening phase, i swung all my forces past the first fort and secured the Union entrenchments from below. Bleed them dry and wait for the moment you can flank the forts. Timing is everything, once you capture the center fort, the right is easy and then you can address the northern previously skipped while putting up a defensive line in the south where the union counter attacks.
Force preservation is extremely important, I had a similar force on legendary...losing something along the lines of 30k taking the forts. With my remaining 35k it was relatively easy to hold both the forts and Washinginton while entrenched.