I need to know how to dis/reassemble a 1974 DT 360?

Your spot on about the manual tombones, the best tool in the toolkit. Got to watch when splitting the cases that you don't knock on the crank with a hammer, one little bend of the crank end (usualy happens on the magneto side, that's the L/H side of the crank on a DT), and its a whole lot harder to get the crank true after replacing the big end bearing.
I find an air compressor and rattle gun handy when re-building 2-strokes..It negates the need for a clutch hub holding tool(I just hold the hub with some rag-but watch those fingers). It also makes short work of removing the magneto nut, the front sprocket nut & the primary drive gear nut!!
Main bearings are often common type & one can often save money by going to a bearing shop for these & on the odd occasion the crank seals also.
Here in Australia quite a few aftermarket parts are available for these. Wisco(forged) & Prox(cast) piston kits,Prox conrod kits,(Prox pistons are marked "ART" in the casting. Just like most genuine *** bikes0.
Same piston, different box, much cheaper!!

Regards

A service manual ( if you can find one ) is what you need. The second thing you need is repair parts. They are pretty hard to find. Babbitt's, a well known distributor, only has parts left in stock for the top end. Go to their website, www.babbittsonline.com/parts/viewbybrand/parts.aspx
and you can see what is left. There's also a diagram of the crankcase center halves. If you can't get the repair parts, spending hours doing an engine teardown is fruitless.

The teardown will require a flywheel puller, a case splitter, an impact driver, and a torque wrench (somewhat optional). Disassemble the top end then remove the left and right side case covers. Remove the flywheel and related electrics, the clutch, crankshaft drive gear, oil pump, kick starter, shift shaft assembly, shift drum end cap and index pins. Now remove the screws on the center case and you can then split the case. This gives you access to replace shift forks, gears, and bearings. Note that crank seals can be replaced without splitting the case.

I am not trying to be a smart *** here, but re-assembly is just the reverse of dis-assembly. This is why a manual would help. Another potential source for parts is to Google motorcycle salvage yamaha .
The normal things you would need to replace inside the center case would be bent shift forks (if shifting is the problem), busted gears, crank bearings and seals, and sprocket shaft bearing and seal. All else can be replaced by removing just the side covers.