do the stock manifolds work for the 350s or do you actually need headers. as far as i understand its the firewall clearance that you have to work with right? i think id be pretty cool to have a hood scoop or a cowl hood to get the carb to clear if thats the reason. i think the biggest thing in the way right now is now having a shop to work in and not really knowing how to weld. plus i've heard getting the sm465 to fit is quite a chore and to get it to throw in the truck is fun as well due to it being such a long throw man i thought my dads old ford had a long throw but this thing in my chevy is trully almost to the dash in 1 and 3 and almost in the seat in 2,4,R which makes a 3rd pass very hard if not impossible.

 

No stock manifolds dont work. They need to fit between the frame rails.
Firewall clearence is the issue. Not sure about clearing the bellhousing on a chevy bellhousing.