Right now I have a automatic I was thinking of installing a four speed. Can I use the automatic cross-member on a manual?

Absolutely!
The auto crossmember will un-bolt...the manual one doesn't! That makes if much easier to work with.
The hard part will be changin out the pedal support so you'll have the clutch and brake pedals to work with the 4spd.

You have decided on a project that will be time consuming.  Removing enough of the under dash to accomplish this feat will be the biggest challenge.  You have to disconnect part of the wiring harness because it weaves through some of the bracketry for the swing pedals.  Other than that hurdle and making sure you get all the parts, it is a pretty straightforward install.  Don't forget to install the pilot bearing in the end of the crankshaft and change the console plate.  The other parts you will need are the shifter (w/linkage), back up light switch, Clutch Z bar and the inboard and outboard pivot balls and frame bracket.  The bell housing (correct size for the flywheel and clutch you are installing) (there are two different sizes) because the 168 tooth flywheel will not fit in the bell housing made for the 153 tooth flywheel.  You also need the clutch fork, and all of the rods.  And, of course, the trans.  Good Luck.

I'm in the process of a rebuilding the engine as well as swapping out an auot for a 4sp. It will take me another two months or so to get the car back together and I will post my progress. I understand that the toughest part of the 4sp swap is the pedal assembly. I suppose I will find out soon enough!

The swing assembly is the hardest part of the whole job.  I highly recommend removing the steering column, left side dash and then tackle it. Once you have that stuff out of the way you will have a much easier time of it.  Don't forget, you need the neutral safety switch, the operating rod and you need to locate the connector for that switch.  Do you have the back drive cable and the bracket that mounts on the trans?  Do you have the bracket and the reverse light switch?  There are a lot of small parts that are needed to complete the swap properly.  If you have any questions, please don't hesitate to ask.  I have done the swap before as well as complete restos on a couple of early C3's and 1st Gen Camaros.

Four of the bolts that hold the swing assembly are actually in the windshield wiper valley.  Two others are above the steering column brackets.  You need to remove the steering column, unless you want your neighbors to here you curse and question why you ever decided to start this project.  I recently replaced my pedal assembly in my '69.  The project is easier if you remove the dash assembly (not required).  My dash was already out due to getting ready for paint.  I only had a couple of wires that were in the way, most ran above the steering column  and in front of the pedal assembly.  I was not converting from one to the other, just replacing a worn assembly.

I did that in my '76.  Its not the same...

Have you considered a Tremec 5 speed.  It is said to be the best $3K you will ever spent.  I put one in my 73 and it is great.

Has anyone made a manual crossmember removable?? I would like to put a 5 speed Tremec in my 73 ---it sure would the job easy!!