The difference between 37's and 42's is significant. The 351 makes plenty of torque and some would argue that it makes too much.
The Weber IDA carb existed on the street for VW bugs. They like the 37's better then v-8's do.
I have run 44mm chokes and you drive them only one way, with the throttle all the way to the floor all of the time.
The 37's are very torquie and you can lay down rubber anywhere you want with them but they run out of rpm's too soon, 5,500 rpm ish.
The 42's are a better compromise for that issue. Mid 6,500 to 6,700 rpm shift points.
I have a set of 40's here that are a bridge between the 37's and 42's. The first time I went from 37's to 42's I didn't like the difference. That's where I got the 40's (actually measure 40.5mm) but they were similar to the 37's, just more rpm, and I went back to the 42's and have stayed with them.
In '78, Inglese loaned me a set of 44's to try. I couldn't deal with that change. There were too many parameters to deal with and getting the jetting right and understanding that took priority.
I would point out that the original 48 IDA was built for the Ford GT40's with 289's and they had 42's in them, so it really comes down to what your preferences are. 44's might be what you want on the Autobahn in Germany? They are a little doggie around town and you need to slip the clutch a lot with them in bumper to bumper traffic. Not a fun ordeal to look forward to.
I will also comment that I have 180° headers with 2" primaries and for me the combination of the 42's, my cam and those headers puts the power curve just about where I want it.
For comparison, when I was first starting out with Webers in '78, Gary Hall insisted to use 32mm chokes on the street. I never tried that but have a pair of them here. They are SMALL, like the venturi on a Holley 1849 carb.
You can see another picture of the carbs installed in my avitar here with 5" tall spun velocity stacks. What those are about is keeping the fuel reversion in the carb. It rises above the stock 3" stacks at WOT but stays within the 5" tall stacks. They also change the sound of the engine at WOT interestingly enough.
The 5" stacks are not included in this sale. Bug Haus sells them for around $100 each but is currently out of stock.
The Pantera seems to have the perfect Weber carb engine compartment with enough air over the stacks to keep them from whistling at speed like they did on my GT350. The vendors call the stacks trumpets but they are a bit more like trombones if you ask me since you can change the frequency by playing with the stack lengths.
