Much of this is pointing toward lean mixture.

You've got solid compression, valves are moving well, etc.

Yes, do use a vacuum gauge. It will tell you if you have a vacuum leak, and it will also tell you if your timing gear is misinstalled (which would be a shocking mistake.)

You've got an electronic distributor timed correctly so no problem there. I'm assuming you've verified that the spark is strong and you haven't missed anything obvious such as plugs or wires or a fouled cap. Rev it up with the timing light attached, make sure the advance works and there's not some weird timing inconsistency happening, then be satisfied with your ignition.

Having eliminated those, you know you're just running lean. Which is enough to cause all the problems you mention. Have you tried a richer needle?

If you know of nobody in the area who can tune dual SUs, find a British car specialist. A person who specializes in Triumph, MG, Austin Healey or classic Jaguars should know exactly what to do with your HIF6 setup. So, in the absence of any classic Volvo people you'll have to hire an MG specialist to dial in your carbs for you. They'll know what to do.

I know it's scary to bring your Volvo to a person who is crazy enough to want to work on British cars... but then, they're good enough mechanics that they can even make British cars run well. Give 'em a shot.