Re: what carb to get or what to do
First off I think that a 850 is too big. How many rpm are you planning on turning? Second I would recomend going with a vac secondary carb, this is an auto tranny right? Neither of those things would cause the problems you are describing....it wont run optimum but it will still run ok. I am going to run a 750 edelbrack on my 454(its been punched out and has headers, and edelbrack air gap, good size crane cam and merlin heads as well as 10.25 compression). I would think that a 750 edelbrack performer would be a good choice for you. An old qget is also a good carb if you get them running right. What kind of intake manifold do you have? I assume you have some sort of squarebore manifold? Demon are nothing more than holley carbs that are made by barry grant, they use holley design and parts(the only thing that wont swap back and forth is the bowls and the fule inlet line) I have heard of many people complaining of the BG carbs having issues with quality controll, people get some carbs and they work just as good as a regular holley, some times they get ones that are junk....The demons look sweet but you tune one the same as a holley 4150 design(or a dominator if you run the king demons) IMO there is no reason to get a custom carb built.
If you want to save money I would just get someone to tune your motor that has some experience. If you have a spreadbore manifold using a qget would work well. If you have a squarebore manifold(or want to change manifolds) I would use an edelbrack performer(probably a 750). Holleys are kind of finicky carbs but can be made to run well if tuned right. Is it a 4150 model?