(2) 4" ports do not equal (1) 8" port,
2*3.14*2^2= 25.12 square inches for the (2) 4" ports
1*3.14*4^2= 50.24 square inches for the (1) 8" port
A single 6" port would give you close to the same port area (28.26")
get a metal bar that size, drill the correct size hole in it, weld plates on the bottom of the new bar that will fit over the existing post like a saddle on a horses back.
get a small monoblock that is rated for 1 ohm or a decent 2 channel bridgable amp that is stable for 2ohm per channel (most are)
Maybe something like this
AB ph1600