Here's the situation: There's a gas line run thru the floor of the kitchen where the customer wants a base cabinet. It's newly run with some of that flexible line under the house and comes thru the floor with a nice flange around it. It's a 3/4 line that has a 3/4-1/2 adapter right about at the floor level and then the 1/2" cut off valve on top of that. It sticks up about 4 1/2" above the finished floor.
This would be fine if the customer wanted a free standing stove but what she wants is a base cabinet with a gas cooktop mounted in it. I need to disconnect the ball valve and extend the pipe by about 6" so it comes up inside the cabinet with the valve accessible. My experience with gas line is mainly with black or galvanized pipe. Does it make sense to go from the brass adapter to a 6" steel pipe nipple then back to brass for the cut off valve or is there a better way to extend the brass fittings?

I know I should have a picture but I wasn't that smart earlier in the day and I'm hoping this is clear enough. The guy who installed the gas line can't get back til late next week so I'm the guy to extend the line so I can install the cabinet.

Bonus question! The cooktop looks to be used and does not have a pressure regulator on it. Necessary?