PCIe is backward and downward compatible. And forward compatible technically.
You can plug a PCIe x1 card into an x16 slot just fine, and it will work. If you cut the plastic 'wall' off the back of an x1 slot, you could technically plug in an x16 card and it would work (just slower).
If PCIEX1_1 is available, definitely use that.
I'd probably put it in the PCIEX16_3 slot if it were me, if PCIEX1_1 is covered up by your GPU's cooler.
Normally PCIEX16_2 will 'split' the PCIe bus assignment to x8/x8 for _1 and _2 on many motherboards when a card is installed there, on the assumption it's a GPU and you're trying to do SLI (don't do this, OBS hates it). You can also install it in PCIEX16_2, so long as you go into the BIOS and make sure that PCIEX16_1 is set to run in x16 mode, and _2 and _3 are set to run in x1 mode.
Not having your GPU running at x16 speeds can cause significant problems with OBS, so it might be a good idea to do that anyway.
Do not plug it into PCI1 or PCI2, those are an older standard. Shouldn't fit anyway.
If you do plug it into one of the x16 slots, make sure it's ALL the way toward the back of the computer. If the pins are misaligned, bad stuff will happen.