OK...Why do you need 3/8" shafts then?  If the existing motors are NEMA 34 then replace them with NEMA 34.  If all you can find is a 1/2"shaft, buy a new motor pulley or bore out the old one.  If you go to NEMA 23 motors, you'll have to make new motor mounts. 

Personally, I give it a shot with the old motors.  It won't cost you anything to try.  If they don't work, then replace them.  You'll use the same drives, power supply and BOB regardless. 

The 3/8's shaft slip into a 1/2 split shaft with a clamp collar on it. The 1/2" shaft the goes through a bearing mounted on the gantry upright. The motor mount accept either the 23 or the 34, no need for new ones. The 34's on the machine have a 3/8" shaft.

Well after a lot of thought I am going to change everything out in this old machine and go with the following. Any thought before I purchase this stuff is appreciated.

PMDX 133 Motherboard
PMDX 126 BOB
PMDX 107
3 Gecko 201's
Nema 34's from Keling
KL34H280-55-4A
New license for Mach
Any ideas on what size power supply that I will need? Will I need a 12v or 5v wall wart to hook any of this stuff up?

The 126 takes a 110VAC supply, the 107 plugs into the option slot on the 126 and needs no separate power supply and the 133 needs no power supply...other than your DC motor power.  Nothing on your list needs a 5 or 12V supply.  To figure your power supply, add up the current on all your motors and take 2/3 of that.  The motor you have chosen draws 5.5A.  5.5 x 3 = 16.5.  2/3 of that is roughly 11A.  Don't go less than 11A.  A KL-4813 should work fine.

The general rule on voltage is to use 10 - 25 times your motors rated voltage.  Your motor voltage rating is 2.4V, so 48V puts you right at 20X.  For lower RPM applications you can stay on the low side.  Higher RPM requires higher voltage to maintain torque at the higher RPM.  I've always tried to stay right around the 20X mark and it's always worked.

Thanks for all of your help!

Does the current set resistor need to be set at 5.5 on the 201?
I just went through the manual for the 201 and found it, It needs to be on 5.6, no 5.5 on the 201

The 201X has dip switches to set the current.  Leave the resistor terminals open and set the dip switches.  Go with 5.4.