That IRS website says it can take 4 weeks after mailing to appear in that tool. That's when mailed in the USA. I would go by your delivery date, and it should be in the IRS tool approximately 4 weeks from that.
"I'm concerned that deadline can expire."
Are you referring to the April 18 due date?
Are you due a Federal refund? If you are due a Federal refund, there is no penalty for a late return. But in any case, your return will not be late if it was indeed received by the IRS on March 8. You also have proof of mailing (I assume). So you can prove when you postmarked it. The IRS goes by the postmark date in deciding if a mailed return is timely filed.
