You all clearly have not dealt with Nick's thought experiments before. It goes like this:
Nick asks a question that doesn't make sense to anyone because it's not how anyone else does it that way. Nobody else does it that way because it doesn't make sense. The bickering goes on for a few pages with people trying to explain to Nick why it doesn't make sense, and Nick insisting that it does. No answer ever comes of this.
Eventually, the project goes nowhere, and everyone is confused by the initial question still because they do not understand that which is Nick. Love ya buddy. In a completely heterosexual way, of course.
So here's my submission:
You haven't specified what regs it will fall under. Limited production/kit cars can have different regs than full on production cars. So assuming limited production, you could probably get away with a Chevy/Ford crate motor legally for emissions, and that will also probably be the cheapest. It will be a V8, but that will probably be cheaper than a 4 cylinder since there isn't a market for new 4 cylinders outside of OEMs, and OEMs will charge you way more money.
Transmission: any of the T-5/T-45/3550/TKO series can be had brand new. Manual trans.
You said the drivetrain will be married to the transmission, which doesn't work except with a front wheel drive setup. Which you won't find for sale new. You're better off buying a new Ford 8.8" or 9", which is also for sale.