I'm going to make an assumption here, so disregard if you are not using a hoist:
it may be possible for you to simply disconnect all the linkages (throttle, steering drag link etc), electrical connectors and brake hoses / vac lines to the engine & trans, undo the rear mount and prop shaft coupling, and let the trans rest on a block of wood and then undo the subframe mount bolts and raise the car on the hoist.... that will leave you with an engine & trans sitting on the floor.
That is of course, assuming you are doing this on a hoist....
If the jacking points on the car are good, the use of 4 old pitman arm bolts, pushed all the way into the jacking points and then sit the hoist pads on the bolt heads. a small test by raising the hoist an inch or so, just to make sure the jacking points are not rusted out and will hold. If they deform you know they are unsafe to use and you have bigger issues - like hidden rust in the sill panels....
This is pretty much how I undertake work on the hoist, when it involves work to the subframes. the main reason being is that you want clear access to everything underneath, and while you can use the front chassis rails, sometimes they get in the way of 'work'.
The funny thing about using the jacking points on the 107's is that the car is just so much more stable on the 2 post hoist, than when using the front chassis rails and the front mount point on the rear subframe....
The big added bonus about doing it this way, is that you now have complete access to all the subframe, for cleaning / repainting / doing all the bushes bearings balljoints etc.
And you can take your time doing it, also it gives you the chance to clean the engine bay, or replace things that need replacing which you would otherwise have very difficult access to.
Also, you can get touch-up aerosol cans to fix any unsightly engine bay blemishes - just wrap up the wiring in aluminium foil (keeps the paint off) and you will have enough access to make everything new.
Oh yeah, the brake master and booster - the R107 booster I believe has a different stud pattern than the early ones... no biggie. the early ones use 4 bolts from memory, and some of the later ate boosters use 2 bolts diagonally. check your 107's pedal box first, you will see the nuts come through the pedal bracket.
I can't remember whether or not it's only the single diaphragm booster or if both the dual diaphragm and singles from the post 09/85 mech update 107's have a different stud pattern.
The 300 should have the same booster and master as the 560sl from memory.
Anyway, it's another one of those 'things' you have to investigate. swapping out a master and booster is much easier when the engine is out. Don't ask me how I know
Post some progress pics if you can - it's good to follow progress with pictures.
It might inspire others as well -