Connecting EV motor to splined shaft

I am in the process of converting a Ford F150 to electric. I am using a motor from a kit (from EPC corporation) which comes with a splined shaft; it is listed as a General Electric 1-5/8" shaft. I measured it as 1-5/8", 25-spline. The driveshaft, on the other hand, uses what I think is 1-3/16" 28-spline fitting (I am using a direct drive since the original transmission was automatic). Where do I look for parts to couple these together? I tried looking at McMaster-Carr but they don't have any couplers with more than 8 splines as far as I can find.

You will need some custom work done.

https://www.hayescouplings.com/hayes-custom-splined-shafts-and-splined-adapters/
Something like this maybe

Another possibility may be to make a 'hybrid' driveshaft, by finding one that fits your motor, and connecting the front half of it to the rear half of the truck's OEM driveshaft. The odds are fairly high, though, that you'd have to have a welding shop shorten one of them, and maybe even end up welding the two halves instead of connecting them at the U-joints...