Mounting shouldn't be too bad....you may find some of the the original holes under the dash, also. There's pics of an '83 and '84 in the Doc, and I'd think the '86 would be set up the same. The radio head and siren were just mounted under the dash. My car has holes in the console delete where switches were probably mounted to control the lights. I still have an old radio mic clip mounted on there as well.
I'm sure the car will be happy to have a drivetrain again.
There sure aren't many restored 4-eyed DPS cars around...