Getting 800x600 is very easy. First, add 800x600 to user_modes.ini or user_modes_super.ini, depending which one you're using.
Then, modify your crt_range like this:
crt_range0 15625-16200, 49.50-65.00, 2.000, 4.700, 8.000, 0.064, 0.192, 1.024, 0, 0, 192, 288, 448, 576
to
crt_range0 15625-16200, 49.50-65.00, 2.000, 4.700, 8.000, 0.064, 0.192, 1.024, 0, 0, 192, 288, 448, 600
That's all.