Always a good idea to get it chronoed mate. Especially if it's been converted or tinkered about with. If done professionally, the power will have been adjusted and some other work done internally for the change of calibre. But I would still chrono it, just in case.
I'm pretty sure that you can retrofit a gauge, but I'm not sure who could fit one for you. I'm sure a helpful BBS member will be along shortly with a suggestion.
EDIT: Didn't notice this question was in the reviews section as it's an old post that came up in my list of subscribed threads. Apologies mods.