perspectiveshifting

Hi, I'm fairly new to this forum and the world of Z's, but I'm hoping for some advice on what to attempt next. I picked up a '73 240z with a large list of modifications, including the SU carbs from a '72, and a "fresh built" l28 engine and head. I've done a small set of fixes and minor modifications to get it running, but as of yet have only had momentary success, and I have a suspicion that the electrical is to blame for the behavior. I've got a Pertronix electric points system installed, with the flamethrower coil that accomponies it, and have confirmed I'm getting a spark. I just replaced the old fuel pump and have it measured at about 3.5psi, checked for any obvious vacuum leaks, put new gaskets on the floater bowls on the carbs, and played with the timing to see if that was the issue, but no luck. I've managed to get it started and idling, but only once, and after no changes, it refuses to again for more than a few seconds. I've tried with and without the ballast resistor, and am going to try some new plugs today, but I just can't seem to pinpoint the issue. Are there any obvious fixes I'm looking over that could be suggested, or common problems I might have looked over? I desperately want to get this beautiful old car back on the road, and would hugely appreciate any and all suggestions. Let me know what information/pics I can provide to help, and thank you in advance.