Dear,
Use the 8 data bit, but programm for 2400 buad, There is no harm in doing so.
U can check u'r code using procomm,
where u set the baud of pc for 2400 with 7 data bit, & on target side(8051 board) use baud rate 2400 & 8 data bit.
Definately u will get the result.
Best regards
Yogesh Kotkar