Ac help and What is this part?

Hey,
In the process of redoing everything under the dash and I’m looking to get the parts I need but want to check with some things. I’ve searched extensively and I will post my findings to cross check. The car is a 1973 dart swinger. Original slant six car with a built 408 stroker going into it. Factory ac car. My evaporator has one of the connections broke off so I’ll definitely need to buy a new one. From what I gather the later 73 models have the connections side by side instead of top and bottom. The only one I can find new is from eBay. does anyone have any experience with these or know of a better option? Second is the heater core, i have read that the new aftermarket ones don’t necessarily fit right. Which leaves me to my original one. The car has been sitting for the better part of 15 years, and its original. So I’m not really comfortable running it and hoping for the best as I don’t want to do this again, from real life experiences is the after market ones really that bad or can it be fine. Or should I take the original one to get re cored and tested and use that? Third. Other than these two main components and changing all the gaskets seals and insulation , would any other parts be recommended to be changed while I’m at it?
Last question for now, is on the firewall side of things what is this grommet with two vaccume lines attached and where could I find a new once. I’ve been searching high and low and can’t figure it out haha.
Thanks a lot!

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