I've found that Tuno, the canned mock tuna from Morningstar Farms,
works pretty well as vegetarian clams in a cream chowder. The chewy
chunks, the vaguely seafood flavor... I imagine it would work with
whatever chowder recipe you have, but I haven't tried a mock Manhattan
chowder. Here's what I made.

3/4 c. diced carrots
3/4 c. diced celery
2 c. diced potato
4 c. light broth, or water
cook on medium heat till tender, 15-30 minutes.

4-5 T. oil or butter (I use mostly oil, with 1 T of butter for flavor)
4 T. flour
1 c. milk or soy milk
1 can of Tuno
salt to taste
heat oil and butter till butter is melted. Add flour. Stir constantly
over low heat till bubbly, continue stirring for a minute or two. Add
milk 1/4 c at a time, mix well after each addition. Mixture should be a
thick white sauce. Slowly add the vegetables and broth, again mixing
well after each addition. Add can of Tuno, mix well. add salt to taste.