Sudden high revving while driving

[Bubbausa]

Hi all, have some questions and seek advice for a 250 inline 6, that has had some changes done…but not done by me.

What I have is a 71 block/engine, a year unknown HEI, a/c, a divorced choke Rochester mv monojet with a rod hooking into arm at top front for choke. It kind of works, more or less, but not real smooth. Carb looks to have been recently rebuilt. It also has a fitting to the intake manifold with a rod that connects up to carb. Running 93 octane non ethanol gas.

Recently while driving I feel the truck picking up speed/surging, so I put in the clutch & its revving at over 3k. Tap the accel pedal lightly and goes back to normal 6-700 revs. So, I disconnected the choke and that mystery rod (heat riser?) from intake. Lubed all linkage, could see nothing binding as I worked it by hand. Went for drive, same thing. Squirted carb cleaner around base gasket, no pickup of speed at idle. Went for another drive, high revs again. Air leak somewhere? Carb issue?

Timing was fine. Was wanting to change plugs and read about HEI requiring a gap of @.045. Looking AT Denso Iridium 5305 pregap set at .044. Correct plugs?

Also want to get new plug wires so I need HEI type 8mm. Any one brand better than the other? Anything else I can check on distributor or should replace?

Thanks for any help or direction you can provide.