I think that instead of using display section of 7135,
you are using 8051 to process results.
If you are careful about the reference voltage stability and
input amplifier errors you can as well get near to 4&1/2 digit
accuracy. External clock like 555 may reduce accuracy by some
few counts but then this trade off in every ADC. Many 12bit
ADCs give accuracy of only 10bits.
To display the counts in six digit BCD I hope you check
this routine:
http://www.8052.com/codelib/HEX6DEC

Here you can convert 3byte or 24bit binary no. to BCD form.
Further there is another utility for driving LEDs from one
port that is 7segments + 1DP. It has a table which converts
BCD no to Display driving.
I hope that after assembling you can drive LED display in
123.456 format.