Clearance between header and ground ???? 1971 A Body

No insults from me, that's a very nice garage I wish I had one. Me, my opinion would order out for a truck load of CA6 gravel dump it in front of the door and start to spread it out. I would estimate about 10 to 12 foot out from the door. Then rent or borrow a compactor to work over the ramp. Then if you have any gravel left, use around the garage's foundation to make it look nice. I cannot tell for sure from the picture you may want to nail boards in between the floor joists to close off the open area under the floor?
If you MIGHT think about going concrete, let “them” worry about it........