Well, since this is a new installation
, you have new, oversized fuel lines, an electric
fuel feed pump, and appear to have double checked the fuel supply avenue, it seems prudent to look in another direction, (though you could try a filter change in the offhand chance that one is defective)
The high idle speed should be about 10 percent over max rpm at load, so 3250 is about right. It is true the tach may be off, you should be able to buy an inexpensive contact or strobe tach for the 80e the shop wants to do it for you.
You might try disconnecting the throttle cable and operating the throttle by hand to see if you get the remaining high idle rpm that way.
If a brand new engine starts easily and runs smoothly without missing, it seems likely that there is nothing wrong with it...though, of course, anything is possible.
I'm still leaning toward something at the other end of the driveline...