The choking is done after the threads. All youll be doing is wasting money and a barrel. Extending a choke out like that is going to blow your choke right out the end of the barrel. Theres a reason we all probably shoot extended chokes these days, longer choking section the better... But the majority of the choke needs to be in the barrel for pressure reasons. You are trying to make an invector choke which was a tiny little short choke (not the same threads as plus), they sucked and rarely threw good patterns with anything. My guns with invector need a full constriction size or more to get equivalent patterns as my inv plus, rem, and mobil chokes.
You probably won't get invector plus chokes, rarely can they do that in a barrel that wasnt already choked that is being shortened. Youll probably end up with tru choke system, hopefully not thin wall because then you cant shoot any nontox. Plus the taps for inv+ are very expensive, not much chance a local gunsmith has one. They are probably going to send your barrel to briley to be done anyways.
