I agree with a solid lifter cam. You already have adjustable rockers. Skip the squishy lifters.
The gears will kill you. 3.73 minimum. 3.90’s would be better. You can’t cripple a 340 with tall gears.
You NEED a minimum of 3500 RPM converter. 4K would be better. Again, you don’t have much stroke. Use RPM to your advantage.
Using Vizard’s 128 formula I get a 109 LSA. That’s too wide IMO. 106 would be as wide as I’d go.
I‘d use a single plane intake if you can find a good one. That’s a Strip Dominator. Or a Victor. They both could use some work but they both work stock.
Skip that Avenger and find a 650 sized Holley clone. That’s all you need with a single plane intake. Any bigger and it will kill all your bottom end. EGT it right and the single plane will beat the dual plane everywhere.
Spend money on GOOD headers. Assuming an A body and power steering you are pretty much stuck with a Doug’s 1.625 header. If you have manual steering and IF a you can find them, Hooker 5204’s are the best middle of the road header.
I would have Howard’s grind a cam with the FM2383719A lobes. That’s 266/266 @.020, 238/238 @ .050, .558 lift on a 106/106.
To make an honest 400 with a 340 takes some RPM. 6500 with decent parts is Stu-paid easy IF you gear and converter it correctly.