I had a hard time understanding the shapeat first, looked like a big turret on top of a landraider. After looking at it for a minute I believe you are going for a stompa on track type of vehicle, reading the 2 black cannons as eyes, and the grey section on top of the the cut blue section as the head.
If so I personally would exaggerate that more to make it more readeable, adding a jaw around the front of the head and pushing a bit more the silohette to read more as a walker on track.
Although the lower melee arm seems cool, I think it breaks a bit the look and symetry of the model, making it too right side heavy (and probably a bit annoying to place other units next to this vehicle.) Maybe it could work a bit better really close to the main melee arm, or placed vertically over the front door or on the back.
As a first overall shape I think it works quite well, and the next pass of detail will definitely make or break it. I recommend doing some drawings of different sections to come up with ideas and have a reference to follow in case you get a bit lost. Making the landrader feel more orky is a must here, since its a quite easy to recognize vehicle.
Both arms might need some work to not look just like vanilla turrets attached to the build. The gun arm has some nice details particularly on the ammo feed, but I feel the fact that it seems to open up as a tank turrent would breaks the illusion that is a massive gun.
The melee arm might need some added bits on the excabator head to make it look like a klaw or some type of ork melee weapon.
I do like the section on the back right, where it seems to be a little platform where some grots could be arguing, with a door and some railings to allow the crew to come in and out or move around the vehicle
I find ork kitbashing and scratch building a bit of a hit or miss, in the sense that is easy to pull stuff together but is hard to make it work well and not look like a glorified glued mashed toy XD.
Looking forward to see the progress, and if you are looking for inspiration, I recently scratch built a couple of big ork vehicles and will be building more on the future, check out my blog