Correct me if I'm wrong, but I think if you set
Thunderbird to connect to gMail through IMAP you should be able upload the message folders; although you might need to do one folder at time depending on how large they are. This would allow you to keep the original received dates as well as the original senders for each email which would be lost if they were forwarded.
Found this article about
imap importing to gmail, one poster had 27,000 emails that they were able to transfer, although it looks like with some aggrivation