You might have some other problem going on so don't assume that drilling the throttle blades will solve it (or do anything at all).

At 378 cubic inches you shouldn't need to drill the throttle blades. That isn't a really big engine so it shouldn't need a ton of air at idle. If you have both sides open 0.020 on the transfer slot you should be okay. If idle mixture screws are all out 1.5 turns then it should run. If it was me I'd borrow another carb first just to see what happens. An engine like that should be pretty happy with a box stock 850 or anything close to that. The carb you have should work just fine too. You might want to take your carb all apart and double check everything. A metering plate gasket on backwards can really mess you up as can a missing jet somewhere or something like that.