LD 340 on a 318?
IMO, stick with the 2176. It is a small port intake.
This isn't necessarily true. The 2176 currently shows as a Performer 318/360, and has the larger 360 ports. I believe that there used to be a Performer 318, which had the small ports, but I don't know if it shared the same #.
I have two Edelbrock 2176 manifolds, both have 360 ports, sort of. 318 intake ports are 7/8"x 1 15/16", 360 intake ports are 1"x 2-1/8". Both of the 2176 manifolds I have measure 1"x2" on the ports. On the other hand, the Edelbrock Air Gap I have (7176) has ports at 1"x 2-1/8". And, just for grins, the old Holley 318 Street Dominator I have has ports at 15/16" x 1-15/16".
So, the 2176 does not have 318 ports. They aren't quite 360 ports either though! I'd bet the LD 340 has 1"x 2-1/8" ports to match the larger ports better.
But, either way, you're still going to have a step in the intake even using the 2176. The old Edelbrock Streetmaster 318's have 318 sized ports, and so do the old Holley Street Dominators.
And as far as 318 vs 340, I don't care what the stock numbers say, if you've got a 340 that's what I'd be running!