Yup, the above article is great. I did just that to build the M54/M113 guntruck King Cobra. It wasn't too bad to do. Build the M54 as Dave describes, also cut the side and front troop rails off the bed. On the M113, build it as per the directions, but leave/cut the roadwheel arms off. I used an Academy M113A1, so I was able to leave them off. You would have to cut them off a Tamiya M113. I also stripped things that would have been removed from the M113, like head/tail lights and guards, engine, exhaust, driver's seat, instrument panel, heater, etc. Use two of the turnbuckles from the shelter on the M925 kit to secure the M113 to the bed, and you are set.
Here is how mine turned out.
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here.