No, this is definitely not possible.
First look at the motherboard differences:
The heat pipes of the G74Sx come together in the middle, while the G73Jh has seperate them to left and right. Another thing you may notice, the G74Sx has a GPU soldered directly on the motherboard, so no chance that you can use your G73Jh GPU heatsink.
Also the screws are in completely different places. The next problem would be that the G74Sx has all connectors (USB, HDMI,...) in different places compared to the G73Jh.
What you can do is swaping your GPU with one of the newest models (7970m,...), search the forum, there are enough posts about this procedure.
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