Help me Hank? I lived in Somerville & Dorchester for a bit - not long enough for the Massachusetts to leave a permanent mark, but long enough to know that turbo-urban isn't my bag.
I guess what needs to happen here is for Subaru to say that the ONLY way for your damage to be caused is from one or from the other - towing douche or engine install douches. Once Subaru says that (and it sounds like Singer has already said that it was the engine install douches), then you might have to go after the guilty party in small-claims court.
I don't know enough anything about transmissions, so I don't know if it's just a question of replacing a part, or if you have to replace the transmission completely. If it's just a part replacement, offer to go halves with them - you buy the part, and they do the work for free. If it's a new transmission, I'd think about small-claims.
Your case isn't really helped out by the fact that you don't have a receipt, however if you didn't get a receipt, I highly doubt that you signed anything absolving the guy from blame if he borked the job. You paid someone for a service that they failed to perform: to install an engine correctly. To the extent that they failed to perform the service, you are indeed owed something.
My $0.02, if it's even worth THAT much.