Without being able to see your detailed route I'd suggest you make the distance a little longer (say 1050 miles) to avoid any problems when ironbutt.com confirms your ride (assuming you care). As you know you need to document the "corners" of the route (gas receipts, etc). If you're not careful you could screw yourself by not having a few extra miles to play with. Let me make an exaggerated example.
You plan a route from Zephyrhills to Ellenton (south of Tampa off I-75). Quickest route based on MS Streets & Trips is 61.1 miles via I-75. If however you took I-275 it would be 82.1 miles. Without a receipt from St. Pete, you'd be short 20 miles at the end of the day. As I said this is an exaggeration, but over 1000 miles, 5 miles is cutting it a bit close. You don't have any input, other than your receipts, as to how you mileage will be verified.
Keep me in the loop.