Looks like a spool of Belden (not Belkin) 7503A, which is 75 ohm RG6 with 20 gauge stranded center conductor. You're really better off with solid center conductor for a run of any length over 10' or more. That being said, RG6 is very good for receive-only applications, and yes it would definitely be better than RG58, which is very lossy, especially 50' worth.
Since you already have the RG6, give it a try, you really have nothing to lose.
Also the "impedance mismatch" means absolutely nothing for receive-only wideband. It adds no measurable loss across the frequency range a scanner can tune to.