If you do run cross tyres on the 5 spot, you might want to harden the suspension up, otherwise the bike will be so low that you’ll be bashing the pedals all the time.
I’ve never had much success with small skinny tyres on a mountain bike, I find it made the wheels feel so small that they didn’t roll over even simple bumps easily. I found larger tyres faster, even with the added weight.
I don’t know the frame, but I’m imagining that you’ll struggle with clearance on your road bike, as others have said. Even if the frame has clearance, you’ll struggle with the brakes unless they’re cantis. Clearance is an issue with real cross bikes when it gets probably muddy.
I’d go 5 spot with xc race mud tyres.