Update on Gravel end on Allez Sprint:
Continental Terra Speed 650b 40mm, actually measure 39.9
mm (on 25mm internal width 650b rim) and it fit on Allez Sprint disc too (but low clearance in the back). So maybe 40mm front and 35mm rear is the way to go.
Had some rides and it work just fine.
What work in my favour:
-Allez Sprint Disc's bb drop is conservative 69mm, not 72-74mm like newer crops of road bike, so it is not too low to the ground when using 650b wheel size.
-I'm using 165mm crank, so ground clearance is not an issue.
-Size 49 has higher trail number than other sizes (slacker handling). This make it not too twitchy on gravel.
-Down tube aluminum is not paper thin, and not easily damaged with flying rocks.
What is not as good as my actual cross bike:
-Short wheelbase, toe overlap.
-Mud clearance. It clear dry dirt just fine. But mud is too much.
-Fitting. I set my fit on Allez Sprint more toward TT position (high and forward saddle, long low bar posision). While this make rider more aero, I put more weight on front wheel and less on rear wheel than i'd do on my actual cx/gravel bike. This, couple with narrow 38cm no flare road bar make people riding with me hesitate to ride too close to me on gravel
So, on the slow ride with full gravel, i use my actual (cheaper) cx bike.
On a mixed (dry gravel and road) ride, I can enjoy my higher speced Allez Sprint which perform well on both road and gravel.
Edit: Re measure, and it was expanded since i first mount. So i edit my post. Still no tire rub despite the expanded width