The cross member is also fixed for manual, but still can be done both ways with manual tranny. Here is how I do it.
Its best to install as a complete aligned unit, but more tricky and tight to slide in . You need to remove the radiator support, you will need a very good engine lift with an extra long boom (big gap to cover between the nose and the final position of the engine in the bay), and you will need a engine leveller to adjust the angle of the engine to guide it into place. You also need good ceiling clearance because engine and tranny will need to be angled to slide in. Install motor mounts but do not install the exhaust manifolds, fuel pump, water pump, starter, and especially distributor, until after the engine is in place.
Alternately, installing engine and manual tranny separately makes it much easier to drop the engine into place but a little tricky getting tranny in from underneath. One issue is keeping the clutch disc splines aligned with the splines on the tranny input shaft so that you can slide the tranny in from underneath. If you choose to do it this way, I highly recommend you try and get your hands on an old tranny input shaft (I have attached a picture of one I kept from a scrap tranny). They do sell plastic repro ones specifically for use for alignment and torqueing pressure plate but much more accurate with real one, especially for torqueing.
Doing it this way, the radiator support can stay in place and engine should be prepared in same way. If you can't find an input shaft, temporarily install the tranny to ensure alignment of splines on clutch disc. Then remove the tranny prior to installing the engine and keep your fingers crossed that nothing moves during install.
After installing the engine in the bay, recheck the alignment with the input shaft to make sure tranny will slide in properly. Now raise the back of the car to allow you to get under there with the tranny. You will need a small jack just under the rear of the bell housing to control the angle of the engine to help with tranny install.
Slide the back of tranny up and over the cross member and as far back into the tunnel as possible, and adjust the engine level/angle as needed so that the front the tip of the tranny input shaft can clear the face of bell housing and into the opening.
Once in the opening, adjust the jack set on the bell housing to get the engine as level as possible so that you can slide the input shaft through the clutch disc and into the engine. If its not level, it won't slide in so this will need some trial and error, lots of giggling and twisting and a few curses before you get it in, but don't give up! For what its worth, I've done it both ways but prefer installing separately. Do not enjoy the sight of big block and tranny assembly hanging over the car!
Good Luck,