Atmega 8 on Arduino Uno
I recommend to use one of the unaltered Arduinos as a regular ISP programmer for the tweaked ATmega8 ones. The reason is a bootloader requires some of the flash memory on the ATmega8. The ISP mechanism in contrary is built into the chip so you can use all of the flash memory for your user program.

Arduino as an ISP programmer is supported e.g. by the avrdude programmer software.