Need a little more information... Which USB2UART are you using? Which board did you use?
Here is the Old Version (boards.txt)
atmega8.name=Arduino NG or older w/ ATmega8
atmega8.upload.protocol=arduino
atmega8.upload.maximum_size=7168
atmega8.upload.speed=19200
atmega8.bootloader.low_fuses=0xdf
atmega8.bootloader.high_fuses=0xca
atmega8.bootloader.path=atmega8
atmega8.bootloader.file=ATmegaBOOT.hex
atmega8.bootloader.unlock_bits=0x3F
atmega8.bootloader.lock_bits=0x0F
atmega8.build.mcu=atmega8
atmega8.build.f_cpu=16000000L
atmega8.build.core=arduino
atmega8.build.variant=standard
But, if you used Optiboot you are going to need to create an Atmega8-UNO entry.
Pictures of you project can be helpful 