What's the general difference in the carbs?
It would seem to me that if they flow enough air and are jetted correctly, they should be similar. But that clearly isn't the case.

There's a lot more to it than CFM and jetting. Jetting only changes the overall fuel curve up or down. Air bleeds, PVCR's and emulsion (along with booster feed size and design, etc.) change the shape of the fuel curve. That's where a lot of the power is.

Like I mentioned previously, an 850 picked up a bunch over the 3910 on my 496, and a 1000 HP main body conversion on the 850 picked up even more.

In contrast, my trusty "mule" 3310-1 780 Holley was within single digit power levels compared to a 950 HP on my 302.

Generally speaking, any "tweaks" I perform on an OEM carb during servicing are mostly limited to cleaning up the idle and off idle transition along with a small bump in primary jetting (application specific). It would involve a lot more time--and subsequently cost--to "race" tweak every carb that comes across my bench. If the carb's going on a driver, it's not worth the extra time & cost. If it's going on a hot rod where every HP counts, then it may be worth the extra effort.

My engine builder says that vacuum carbs don't dyno well compared to mechanical carbs. Any opinions?

AND what makes HP on a dyno may not apply to race track conditions ... just sayin' ... TAZ

This is 427 Yenko engine same cam & lifters, +.030 TRW pistons & parts as the 396.
One dyno pull, ignore the figures bellow 4k RPM vac sec acted up.

Dave Beems L/78 made 450 STP HP on my dyno with my carb and headers, all factory parts but .030 over on the pistons.

If any one is interested here is a dyno sheet from today.
Original Yenko L/72 .030 11to1 speed pro pistons Comp cams replacement L/72 cam 93 pump gas shop headers.

L72 dyno numbers...
 

Impressive :biggthumpup:

68 nova ss 396
 

recently I dynoed my 396/375 hp motor for my 68 nova ss matador red car...with lots of tuning through stock manifolds and 520 lift flat tappet camshaft best hp was 465 hp and 455 torque.. Chris teed

L78 Horsepower..
 

Chris...those are very strong numbers with cast iron manifolds....nice job.

-wilma