If it's already had a barrel extension installed, and the gas port hole drilled, then about your only option is to torque the new barrel extension on until it lines up with the old index pin hole (TDC, and the TDC for the gas port as well), and check it to see if it headspaces correctly.
If it doesn't, you're SOL unless you want to weld up the old gas port and front sight base grooves, and take a chance on torqueing some more (or backing off) on the barrel extension to get the headspace right, and hoping that there's enough room between the face of the barrel and the back of the extension for the bolt to fit, rotate and lock. If so, you'd then need to drill a new gas port, index pin hole, and then drill for the front sight base taper pins again with the new TDC.
If the chamber's been chrome-lined (as almost all Colt barrels are), then you'll screw it up trying to shave the face of the barrel and re-ream to correct depth.
Basically, unless the new barrel extension goes on and headspaces correctly at the old TDC, it's probably more trouble than it's worth.