You need:

Pylons
Rockets pods
Wing
2 Fuselages
Fuselage inlet

Fine Tuner

Step 1

Put the pylons updside down on the wing , and put the rocket pods on it.
Attach the wing on a fuselage
Now, to make all the rockets works without exploding , you need to programm the activation group for the pylons/rockets

Pylons (1st,2nd...) : 1, 2, 3...
Rockets (1st,2nd...) : All, 1, 2...

Step 2
Attach the fuselage (with the inlet on it) on the other one.
Use Fine tuner to adjust the duo pylon/pods with the inlet (you can't put the inlet all along with the rockets, it'll explode)

Step 3 (end)
Put rotators, and when you'll need to reload, put the gun nose up or down and you'll press the [1] ; It'll eject the first barrel. Same apply to the other ones ;p

If you followed those steps, here is what you will have :
https://www.simpleplanes.com/a/2DX53c/RTTF-01