You might be best of to get rid of the Mac on craigslist and switch to a PC.
I see no sense it sinking money into an old computer system. In about 7 weeks you will be able to get 6 core Coffee Lake CPU for about $350.00 and a GTX 1070 for about $420.00. For about $1,000.00 you can have an awesome PC. Simply use the storage and monitors of your Mac Pro for the PC. Keep in mind the i7 CPUs have been able to outperform the XEON CPUs when using Premiere Pro. Also the GeForce GPUs have been able to outperform the Quadro series cards when using Premiere Pro.