The head has a fast burn characteristic applied to the chambers in an EFI head. It'll work with the carb, but I can't remember if it's a benefit or a detriment to performance. If it's a detriment, it's not very much, just not optimal.

The head uses 16 bolt holes on the EFI and 13 on the carb'd engine. The 3 extra bolt holes are only for ease of installation, since the EFI manifold setup used 3 pieces, whereas the carb setup used 1 (intake and exhaust were solidly bolted together). This is a complete non-issue.

The EFI's exhaust manifolds are a common bolt-on performance upgrade to carb'd engines. They may require a slight amount of grinding (and I think one of the exhaust manifold's hanging tabs needs to be removed) so that it doesn't bind with the intake manifold. It's not a big deal though.
Other than that, the intake manifold will bolt right up like it was made for it.

The distributor will drop right in.

I *think* the EFI engine uses the same cam as the carb engine.

The biggest issue is what what Muscletruck said. There is no provision on the EFI block for the fuel pump. Not a show-stopper, but something to be aware of. An aftermarket electric pump/regulator will solve it.

The 300 will be able to tow pretty much whatever you throw at it. It just won't do it quickly. If you want to increase the power, though, you can always look into putting in a performance cam and getting yourself a 4bbl intake/carb to bolt onto it. Find yourself a good carb'd 300 head and do a good port and polish on it. It's amazing how quickly the power of the 300 can be boosted.

The 300 uses the same smallblock bolt pattern as the 302 and 351, so any transmission that bolts to those will bolt to the 300.

Gas mileage varies greatly with the 300 from 12 - 20. It depends a lot on vehicle weight, gearing, tire size, etc. The C6 isn't exactly the best on gas either.