Did you swap out the throttles to ones without the overrun flaps? One of those could be weak.

Next mod I do to SU's in convert them to fixed needles, then knock the hat off the BBB needles & sand off some knurling to make them slide in neatly.

You need longer carb studs + plastic spacers.

You need to loose the steel fuel pipework because the fuel may froth up in it from vibration.(There's a reason Volvo used rubber hose from the pump up to the carburettors)

You probably need to cut your ignition centrifugal curve down & increase the static timing so that the idle throttles are more closed at idle. It depends on compression & how big the cam is. John should be able to tell you how much total timing your engine needs because it's his head & cam. If he says 30, limit your distributor to 10 degrees & use 20 static.

I don't think there's anything wrong with HIF's. They flow about 15% more air than HS6 SU's + the go around corners a lot better & brake better. They can be fussy with needle & seats. I like the pink rubber tipped with damper spring type. Solid one's don't like to be shaken up by a 4 cylinder.

Forged pistons? Are you going to add a blower kit to it?