There's an interesting thread on Lonely Planet about this, but it is from 2009.
Another option.... there are travel companies that do this trip. here's one I found.http://www.dragoman.com/holidays/details/overland-between-beijing-and-bangkok
They have a 62 day trip. If you don't want to go on an organised tour maybe look at their itinerary and route for ideas to travel independently.
From what I can see from others who've done this, 5 weeks might be a bit of a rush. can you spare any extra time?
And if you plan on doing any of the driving yourself (even on motorbike) you'll afce some challenging road conditions and driving standards. If you're doing it by train and bus, again there are some safety issues.
All of these topics are covered in various safety guides on WorldNomads.com, have a look at these pages:
Phil from the safety hub about 7 years ago