Drop-in, I'd just suggest getting a high quality liner. I've had several "Ford" liners and they never warp, but the one time I went with a cheap liner it warped. As for spray liner, Line-X is good...just make sure an idiot doesn't get behind the nozzle - last month I had two Ram trucks in my shop with Line-X liners and one looked new and the other was a putrid mixture of bubbles and peels. Never had a problem with Ford drop-ins in the last 21 years, see no need to ever change.
'08 Vista Blue XLT 2.3 DOHC
'11 Suzuki S40 - SOLD (5-3-2012) - Newest '12 Yamaha XT250