Im not quite sure what your asking ??
Is their a safe way to tye into your gas main ?? - Since you bought '3/4 black pipe' Im quessing your main is steel , Do you have a union on your main?? If not you will have to disassemble the connection to another appliance and somewear at or near the main install a T but you must not undersize the pipe becouse now you have 2 or 3 appliances running off the line
The second part of the question , Im quessing you bought a pice a pipe and had it cut and threaded to size, so your woryed about the lenght of the pipe extending enouth thru the floor ??
Are you aware they sell atleast 2 types of flexable gas pipe for use inside the home - Trackpipe and Wardflex bolth not cheap compared to sch. 40 steel but is alot easyer for a homeowner becouse you dont need to cut and thread pipe , its a simple compression type fitting that is legal most everywear
If your useing propane you can always run K type copper tubeing
Some other hints
If your appliance is not permentaly mounted make sure to use a flexable gas connector
Before your appliance always put a valve ,union and a dripleg/dripcap
If possible put a pressure check on your work , if not soapy water will work to check for leaks just double check your work for safty
