I'm working on a Biber too!
Funny, your comment about making the model functional/positionable. I'm doing an OOB build with the only mods being the same that you are attempting.
Seems Revell made some parts movable, and others not. THe back support (for lack of a better name needs help.
Here's what I've done so far (forgive me as I don't have an electronic camera, so I'll have to describe it).
Mods were easy. Careful drilling (I do mean careful) is all it takes.
I only needed a pin vise and several drills. The four sets of "rollers" on the main boom (front arm) need to move. I drilled out their bosses on the arm, and put in a length of round stock, and glued the "rollers" onto the round stock. The round stock is now an axle. The front rollers were the most difficult because they are more visible, and because their mounting bosses are small - drilling them out leaves little plastic around them.
Do yourself a favor, and make sure the the rollers are parallel, aligned and at the correct width when you are finished. This is easy to do... befor the glue has set, place them in the corresponding channel of the bridge (build the bridge before you build the main boom), and make sure the bridge can slide back and forth. Leave the bridge in place until the glue has set.
I repeated the process for the thin support arms on the main boom.
The rear support arm (or whatever it's called) can be made to articulate, too. I found some thin sewing pins, and used a drill of the same small size.
I carefully (I do mean carefully) drilled out the main pivot point of the arm and mount, cut the pin to size and inserted the pin to hold the arm in place. I did the same thing for the lower mount of the hydraulic actuator.
I had to scratch build the upper mount of the hydraulic actuator ram, as the mounts Revell provided were too small to work with. I made some blocks out of square stock, and drilled them out to fit the round stock I was using. I took two pieces of round stock and glued them together perpindicularly, so that they made a "T". Once the glue set, I put the drilled out blocks on each side of the "T", and glued the blocks onto the arm where Revell would have you glue the end of the hydraulic ram. I then cut the ends flush with the blocks. This is really the only place where the mods are out of scale. The mountings are somewhat chunky, but I build my models for fun, not perfection. Once I paint and weather the beast, it won't look too bad (I hope).
As for the hydraulic ram, I drilled out the body of the hydraulic actuator to fit the round stock. This was tricky, because the hole needed to be straight, and once done, the plastic got pretty thin. If I hadn't drilled straight, the drill would have come out the side of the actuator. I started by cutting off the hydraulic ram, and scored an "X" on the center of the actuator with an Xacto knofe. This provided a center for the drill when I started drilling.
I started with a tiny drill and drilled to the depth I thought the ram would go. To keep the hole centered, I didn't turn the drill, but instead, I spun the body of the ram about the drill. Once the hole was made, I used increasingly larger drills until I got to the correct size.
I then mounted the actuator on the tank, and inserted the piece of sewing pin for its pivot point.
A note about the sewing pins: Drill out the holes just right... not too big, because you may not have enough plastic, and not too small, because if you do, the parts will break when you try to drive the pins in place. If you get it right, the pins will be held in place just tight enough that they won't need to be cemented.
Finally, you'll have to deal with the rollers on the rear arm. For some reason, Revell molded the arm so that ti wasn't wide enough to support the bridge sections. Cut off the round knobs at the top of the arm, and glua a piece of round stock that is wide enough to fit into the same channels of the bridge that the front rollers fit into.
Right now, I'm not finished, but it all works well(!). When I set it together, it works, and I see how the real one works. Well sorta. I don't know how the bridge is set in motion. I have a feeling that there is aome sort of cable mechanism that pulls the bridge pieces forward, and my guess is that is why the center bridge girders have holes in the middle of them (nicely molded by Revell). The other thing I don't get is how the front of the rear (top) bridge is held in place when it is in the fully stowed position. It just flops around on my model. There's nothing to hold it in place. I'll go on line and see if I can fugure this out.
If I get a camera and you're interested, I'll get some pictures and send them to you.
Cheers!