Update:

The dual 390 cfm Holleys seem like they'll work well on the 289 Weiand #1988 manifold. We put spark to it last night and it started right up and idled thunderously. It wasn't getting fuel from the pump, though (???) -- dribbling gas into the carbs it would idle for 5-10 seconds at a time and then die.

We're working on the fuel pump glitch but this sounds like it's going to be a runner once we get that dialed in.

Future advice for anyone who finds this thread a year from now using the SEARCH function: Definitely go with the smallest carbs you can find if you're going to run a small-block dual-quad tunnel ram.

Thanks for all your help, everyone!