So, did some reading on this, as I found I had a Ph.D. thesis about the Laird Rams on my HD. (Online? Yes,
here)
At least 13 of the 300 pdr rifles were forged and rifled for the RN, and I've seen comparative tests against the 15" Rodman HMG bought. It is usually said that 12 were issued to ships (4 each to Scorpion, Wivern and Royal Alfred) and one was used for shore tests. Other were forged for other customers (for example, two for Huascar and one for CSS Stonewall). The Royal Sovereign was initially armed with the same pattern gun but not sleeved, and hence smoothbore.
Wivern definitely had 300 pdrs in November 1867, because the left gun of the forward turret burst during target practice. She was taken out of Commission and emerged with 9" RML replacing the 300 pdrs. No-one was injured in the explosion. The problem was the gun in question was one of the early pre-1863 forged guns. A previous gun had failed in testing by the breach bursting in the same manner, and so later guns were reinforced.
Because of this accident the 300 pdr was completely withdrawn from sea service. The remaining guns were resleeved from 10" (10.5" was the "smoothbore" size) to 9" as a variant pattern 9" RML.
In fact, since we're also talking carriages, the Scorpion and Wivern received different carriages. In 1865 there was a shortage of the new Scott iron carriage which were replacing wooden carriages. All the production was already earmarked for other ships receiving the new guns (like Pallas) and Scorpion and Wivern were bottom of the list. It was decided to issue some old wooden lifting slides for them so they could go to sea for testing (they needed the weight of the guns to balance the turret) - hence she got the duff wooden mounts which were noted to be "very defective" when issued. This was meant to be a temporary measure. On 29th December 1865, after pressure from the government, Wivern was issued modern iron carriages with 300 pdrs. I have no data on when or whether Scorpion's carriages were changed before her refit, but they received major work after her first test cruise when it was found that they took an hour to move through extreme elevations etc. One suspects the Scorpion kept her wooden carriages until refit.