Put it in the slot closest to the CPU.
Many MBs (not sure about yours) will take PCI-E lanes from the second x16 slot if you add an NVMe drive or some additional SATA drives. Won't cause much performance degradation but its just the principal of the thing.
Heat really shouldn't cause much of a problem (my 980TI is right next to my OC'ed 6600K and it still boosts to and holds 1500mhz on its own).
If you can get a fan blowing air from the side of the case onto the GPU + CPU with some solid venting you can even keep the fan speeds down.