A few tidbits from my gmt travels that may help -
You cannot convert a 1030 to a GMT, because its pillar lacks the channeling & screw holes to mount the date & gmt modules. Instead, pay whatever you have to pay for a decent, working 1036 or 1066 (these are 1030s with the date module & either is correct for a 6542 - the 1036 was used initially & the 1066 at the end of production). When I say working, I mean make sure the movement keeps reasonable time & the stem winds freely & sets time without *any* binding. This is VERY important as problems with any function on these movements are likely to require (expensive/rare) parts to rectify. I should add that most Rolex watchmakers will refuse to work on the gmt due to the rarity/expense of parts & the fact that the pros cannot spend the time required to source them.
Your biggest hurdle (assuming you are starting from a working 1036 or 1066 base), by far, will be locating the gmt components, especially, the key 24-hour (gmt) wheel & calendar ring (the links below discuss this). Unlike nearly all other components inside Rolex watches, these parts are unique (to the 6542) & were never used in any other watches. Because of this, they were made in limited numbers & only for a brief period of time. Add in the severe demand for these parts from an international cadre of well-heeled collectors & you have a nightmare scenario when it comes to availability.
If you have not already, you should read (in no particular order) this, this & this.