You say you have successfully multiplexed them, but then say you don't know how to connect them.
So how do you know that your multiplexing is successful?

The 74LS373 is an octal latch - not sure why you'd use six of them to "multiplex" six 7-seg digits?

To multiplex six common-cathode 7-seg digits you would connect all six 'A' segments together to a current source driver;
similarly all six 'B' segments to another current source driver;
etc for all seven segments (or eight, if you include a decimal point).

So that's a total of 7 (or 8) source drivers for the segments.

Then you need a current sinking driver for each digit - a total of six current sinking digit-select drivers.

Segment
(Source)
Drivers
         +---------------------------+
         |                           |
  A -->--+                           |
         |                           |
  B -->--+                           |
         |                           |
  C -->--+                           |
         |                           |
  D -->--+                           |
         |                           |
  E -->--+                           |
         |                           |
  F -->--+                           |
         |                           |
  G -->--+                           |
         |                           |
         +---+---+---+---+---+---+---+
             |   |   |   |   |   |
             |   |   |   |   |   |
             V   V   V   V   V   V
             |   |   |   |   |   |
             6   5   4   3   2   1       

          Digit-Select (Sink) Drivers

That certainly means that you can't drive them directly from 8051 port pins - nor 74LS373 outputs!
Remember - the digit-select drivers must sink the current for all 7 segments when displaying an '8' (or all 8 segments for '8.')

As you're going to need external drivers anyhow, you might as well follow Ian's suggestion and get a chip that does the whole job for you:
http://www.8052.com/forum/read.phtml?id=91468