Nice start. The mods you are doing look really good for your first attempt at them. For the vents you cut, next time, get a fine drill and drill the corners, then drill some more holes along the edge to connect the dots, then take your knife and break the small remaining bits of plastic between the holes. Then just sand and file. Will save you a lot of blade tips. For the paint, try to strip the old paint first, since it hasn't been on the car long it shouldn't take long remove it. Personally I like to use Castrol Super Clean, think its just called Super Clean now, can get it here in Canada at Walmart. Big jug of it is like 10 bucks and last for a REALLY long time. I just cleaned out my container of it after stripping my current restoration build and it was probably in the big 4L ice cream container I use as the bath for about the last 1 1/2 years. Others use brake cleaner or alcohol, but I prefer the CSC route. Use a coat of grey primer to find your flaws, then sand it out, coat it in white primer then paint, should hide your grey under primer and give you the white you're looking for.
David