Kansas Kustoms is the only place I know of that does alot of exhaust splitting. You may need to send in your own core though. Otherwise, the cheapest source for headers I've been able to find is at hotrodsofnorco.com but you will need to make or buy a separate system for intake heat.
If you are running multiple carbs, or a 4bbl, you aren't likely to need to increase the jetting, regardless of exhaust. See the tech article on multiple carbs on this site.
As for exhaust sound, we can talk all day about it, but it still comes down to you trying something and seeing how it works. Sound doesn't travel well over a chat room. All glasspaks are loud, period. If a single glasspak/exhaust is too loud, then going to duals won't be any quieter. A crossover pipe will help mellow it some, but probably not enough.You may want to try a cheap "turbo" style muffler instead. As for tube diameter, Fentons can take a 2", if you were to try running anything bigger, you will need to make an adapter like they do for large tube headers on smallblock V8's. But there is no reason for more than 2" on a street 235.
There are sooooo many variables that can affect sound, such as compression ratio, cam profile, idle speed, position of muffler, length of muffler, length/dia of tail pipes, shape of exhaust tip, and many more...
There is a book called "scientific design of exhaust & intake systems" out there that I bought but haven't done much reading yet, but it appears to have everything you could possibly want to know in it, and it may even cure insomnia.
A friend of mine is running a basically stock 235 with dual carbs and dual exhaust in a '52 car. He is running 2" pipes with Smithys, crossover, and about 3" by 10" long tips, and mufflers mounted right under the seat. Has a nice low mellow tone to it at idle, but will still bark above 2500 rpms. He says adding the tips and crossover helped to lower the tone.
as for length of tailpipe and tip sizes, you can experiment rather easily yourself. Get a tube slightly larger than your tailpipe and slide it over it and you can easily change the length of your pipes this way until you find what sounds the best. Then you know where to put the muffler and how long the headpipe will be also. You can do the same with tips, using things like coffee cans, Fosters beer cans or whatever to simulate the tips. What I'm trying to say is it shouldn't take alot of money to come up with something that will sound good to you.
