Built a 40m dipole using 14AWG stranded wire and calculated total length at 66 feet using the 468/f formula. Using 468 divided by 7.2 MHz center frequency gives me 65 feet total. I'm getting high SWR (3:1) across most of the band with my LDG Z-100 tuner. Question: I made my own center insulator from a small PVC junction box and SO-239 connector. Could the insulator design be affecting my impedance match? Also, should I be using a 1:1 balun?

Your center insulator design sounds fine, but definitely add a 1:1 balun to prevent RF on the coax shield and maintain proper radiation pattern. High SWR could indicate your wire is too long - try trimming 6 inches from each end and recheck.

It's easier to cut wire off than add it back, so always start long. Your PVC center insulator is probably not the issue. I'd check your coax connections first and make sure you're measuring SWR at the antenna feedpoint, not just at the radio.

Had similar issues with my first dipole. A commercial balun like the LDG model runs about $30 and includes current balun protection from RF on your coax. Worth the investment for consistent results. Also verify your frequency - 40m phone and CW segments need different resonant points.