Although not as user friendly as ľAN, you should be fine using NPT connections for this application. The majority of fuel pressure gauges and senders out there are NPT.
A lot of the aftermarket fuel pressure regulators with ľAN line connections still have NPT gauge ports. There are also regulators out there (like those 100 psi Mallory adjustable regulators IIRC) that have NPT line connections.
In general the flare fittings are best used for fuel line connections (flare fitting hose ends are easier to install and less bulky than NPT hose ends).
To make NPT connections I first make sure threads on both parts are clean and free of defects and then wrap 2~3 layers of Teflon tape tight and neat around the male part. To test for leaks, run the fuel pump to bring it up to pressure without starting car and let it sit for a while.