You got me curious, so I did a google search. This is an online pic of a 71 Dodge 340. Looks like the sending unit goes into the outer port...This a small block? I'm not sure, but something don't look right. I don't think there should be 3 heater hoses. Are you sure one of those ports isn't for the temperature sending unit? Just guessing, as it's hard to really tell whats going on from your picture.