If the US 76mm would be too much for the Chaffee turret, then the 77mm gun from the Comet should be, as well. I believe the 77mm was the more powerful of the two, so the length of the round and the level recoil would likely be greater, as well.
The 25 pdr may work, and the 95mm howitzer almost certainly would, too. Now, if you modified the Chaffee turret Firefly-style, then the British 77mm may work, but I won't be volunteering to load. You may have to go down to a 2-man turret Valentine/AEC-style, but British recon units weren't unused to that compromise, so I don't see that being a deal-breaker.
The Chaffee's M6 75mm gun was indeed a variant of the lightweight gun developed for the B-25G/H. As I understand it, they used the same round as the M2/M3, so the recoil forces should be similar, but the actual weight and bulk of the gun was considerably less. Now, obviously, I'm sure you could substitute a 6 pdr for the 75mm if you had a couple of competent fitters and a lazy afternoon.
The most powerful gun (recoil-wise) that I recall having been mounted in a Chaffee turret was probably the Israeli 60mm HVMS on the Chilean Chaffees.
Cheers,
Logan