Hi,
my holes are down around the heel of your foot on the inside wall and I use a horse syringe, a big body with a small'ish nozzle, and with the Xtroll product I use it goes in quick and would spray about a bit. For a bit more discretion you could put some holes closer to the set cross member, but I found it will seep out of any gaps all along the length of it.
I wouldn't be keen to seal up everything from underneath using a body sealer as it will mean that anything that does get in, including condensation, has no way to get out but to rust.
Terry