That is likely a tuning issue. My guess is that she is running too much Idle-timing, and if you close the Idle speed-screw, she wants to hesitate at take off.
The cure for both is to run less Idle-Timing.
If you brought that car to me, I can make it idle at 550rpm, in gear and pulling itself on flat/level/hard ground; with any cam up to a 292/292/108, the biggest that I have tried. The only tricks I will use are idle-timing of 5*, and a couple of tiny holes in the primary throttles. Plus
If the piston ring-gaps are too tight, I will change that.
If the engine consistently runs over 210, I will change that.
If the carb is sucking hot underhood air, I will change that.
If your WP speed is too slow, I will change that.
Leave it with me, and when you get it back, it will do what I said it will, even if I have to pull that crummy cam outta there.
There is no issue with idling at 900 with a 4-speed, except you can't drive very slow. If the car still has 3.23s and a 2.66box, then the slowest speed she can drive is about 8 mph@900 rpm. which sucks. If you want to drive slower, you gotta ride the clutch, and in the meantime; with excessive idle-timing and a nearly closed throttle, the doggone car is getting jumpy. The cure for that is less Idle-timing and more throttle. And if the engine is unhappy, it is because she wants some bypass air.
One size bigger Mopar cam, to me means the 276/284, or possible the 284/.484 cam; both of which can be had in LSAs of either 108 or 114s, and in 110s aftermarket. So there's a lotta variables right there.
As for Idle-timing, that 340 will like big numbers, in the range of 25>30 degrees. But, as you are finding out, you can't run it there with a 4-speed.
Try 12>14, and reset the speed-screw/mixture screws accordingly.
Make sure;
>the secondaries are fully closed.
>the PCV system is functioning as designed.
>If you have a metering-rod carb, make sure the rods are staying down.
>If the idle is not smooth, Temporarily introduce a bit of bypass air as may be required, until the mixture screws like to be in the center of their working range; about 3/4 turn on a Holley-type, or 2.5 on a Carter-type. I do this into the PCV hose or into the PCV itself . Once I got her running sweetly, I measure the hle I drilled in the PCV system, calculate the area, devide it in two, recalculate for two holes, then drill each primary T-valve the appropriate size. I put the holes on the transfer-slot side, between them and the Idle-discharge port, about 3/16th from the edge. I usually drill 1/32 smaller on the first test. Then go back and plug the hole(s) drilled in the PCV system.
If you drill the throttle-holes too big, don't panic. Chamfer the holes, warm them up and solder a slug in there, making sure it overlaps the chamfer nicely. Then dress it down just a lil.
I have not had much success drilling the solder plugs, so I just move over a lil, with a smaller hole this time.
Some guys will/may tell you that drilling the plates is the wrong thing to do. Lemmee tell you a wrong thing to do; wiping from the back.