A really good conversion (and easy too ..!) would be to get the "Stuka ZU Fuss" kit, take the rocket assembly from that kit, and put it on this Halftrack. This was the more common version of this, the Sd.Kfz 251/1 28cm rocket launcher. Both are Tamiya kits, and that makes them "shake and bake", and the early Tamiya 251/1 C is OK. Remove the rivetts, do the weld-seams with a pirogurveur (SP?), and this makes it the late "C" version. Don't forget to place PE treadplate on the floor where the driver and the co-driver sit. Read the articles on AFV Interiors on the 251, and the Squadron/Signal book as well for further info
Or you could do a search on the net, for "Sd.Kfz 251" and print out the pictures of the 251/8 ambulance version, as this is 80% of the Tamiya kit OOB.
This exact kit-bash, as above, is on my work-bench right now. (251/1 /w 28cm rockets)
Rob Savage