The board looks fine to me. 75 Ohm on the outputs is correct for an NTSC console. All three capacitors C4,C5,C6 of the color outputs are bypassed as they should be. The big capacitor on there is likely a 100uF power filter. It is next to a smd cap and the schematics of the 7314 recommend a 0.1uF and a 100uF capacitor between power and ground.
TTL CSYNC out of the multi-out is also correct. All NTSC SNES and N64 CSYNC RGB cables should have a 330-470 Ohm resistor on the CSYNC line.
An official PAL RGB Gamecube cable uses composite video for sync and it has a 75 Ohm resistor to ground and a 220uF capacitor on the sync line. Composite video sync and luma sync cables with NTSC consoles should have no components on the sync line. As your Super Famicom also works with the cable it may work with the N64 too. The only thing that can happen is that you can get unstable sync because the resistor on the sync line lowers the voltage. As long as it works it is safe to use with the OSSC.
If you have sync issues or see diagonal interference in the image then its best to use a NTSC luma sync RGB cable.