Right now, you only have 1/4 the proper conductors (2 wires instead of 7/8) and that'd make me a bit nervous. If this PSU was my only option, I would cut the SATA, crack open the PSU housing, and build a new 8 pin from scratch, but I know most people don't have the tools for that. You could go the inline solder route and daisy chain the plug end, but it's one of those things where if you don't have the right tools, you probably don't know what you're doing and you probably aren't going to do it right. Messing around inside a PSU exposes some dangerous voltages you shouldn't be casually messing with.

I would also worry about the OEM PSU having enough oomph to run the GPU though. They aren't usually built with a lot of overhead or high end filtering, so you could end up sending dirty power to the entire system and start killing hardware or flipping bits in unpredictable ways. Getting an SFX psu and repinning the plugs would be my go to solution unless the OEM PSU has some really weird hardware or unsourceable cimp pins.