Baker,
Beater said it just right.
I'd add the following, you could do this first or in any order really:
* Get a tach in the car if you dont have one already, you need a tach with a stick anyway. Your combo should be shifted at about 5000 or so (when its running right), but you dont want to guess and take it to 6500 instead (BANG, oops.....)

* Take some readings cruising down the road in say 1st-2nd-3rd gear, speedometer vs tachometer:
Heres what I get:
Speedo-----Gear------Tach
20----------1-------2200
30----------1-------3300
40----------1-------4400

20----------2-------1200
30----------2-------1900
40----------2-------2500
50----------2-------3200
60----------2-------3800
70----------2-------4500

20----------3--------800
30----------3-------1200
40----------3-------1500
50----------3-------1900
60----------3-------2300
70----------3-------2700

Those should be close, but note what you get (Better yet, FILM the tach & speedo so you can play em back together later....)

Then, film the tach and speedo together going thru the gears.
The relation of tach vs speedo should be the same.
But I find that with many 50-year old speedos, the speedo will LAG under acceleration. Some tachs will do this too so this isnt foolproof.

One hint is if you let off the gas and the speedo suddenly jumps UP, its catching up cuz it was lagging during the acceleration run.

This is why I use a Gtech instead of speedometer+stopwatch especially with these 50-year old units.

Weigh the car too so you know what youre up against, but that is just a detail compared to the current dilemma, Im sure its 3600-4000, somewhere in there.

In a flat parking lot, try pushing your car. With a stick and 30+psi air in the tires, you should be able to push it easy yourself. If you cant, or it feels like the brakes are on, well, maybe they ARE on.

After doing all this and what Beater said, I would then:
1. Check to see how that cam was installed - is it a tooth or two off? That could cause what youre seeing. You might get a hint of this in a compression check too.
2a. If you can borrow or swap-meet a couple known good carbs, try them on it. I have at least one old BC carb thats just a DOG, no matter what. You need spare carbs anyway!
2b. Put a single carb iron intake back on it, and then try each of your carbs, or even some borrowed known good carb.

But until you do all the Beater/tach/speedo/weight/push it stuff, I wouldnt dig into the motor cuz theres gotta be something WAY off here. Good luck, we will figure it out.