When strapping down top heavy machines like a lathe I always:
Start the strap by anchoring it on one side of the trailer, go over the top of the machine, wrap the strap around the whole body of the lathe by looping the strap underneath the lathe before going over the top a second time and securing the strap to its second anchor point on the other side of the trailer.
Just running a strap over the top of the machine will keep the machine down on the trailer but the loop around the whole machine keeps the machine from slipping underneath the strap allowing it to move around and possibly fall over.
Of course you will use multiple straps and the loops will keep the lathe from being able to fall over underneath the straps. The loops almost eliminate shifting on the ride home even when the machine isn't top heavy. This technique has always worked very well for me even on very top heavy things like a stand up vertical compressor with the huge motor and multistage pump on top of it.
Like others have said bring blocking to keep pressure off the lead screw etc. An engine crane (cherry picker) is one of the best investments that I made and great for unloading.
I have had a few sellers (even a couple old salts) watch me make the loops around the body of the machine with a "that makes a heck of a lot of sense" look on their faces. LOL!
Here a single loop (I don't know why I went under instead of over the RAS arm) kept it upright for a few hour drive home. You will have to look very close to see how the strap is looped around the arm. Like I said I usually loop over the machine before going around the machine but in this case looping under worked just as well. Without the loop this RAS would have fallen over when I went around the first curve or did the first quick stop.