Here are some great threads, I encourage you to have a good read through them, ears are the worst and do take a considerable amount of time and care to get right, adding a step also helps them to sit a little flusher.
Some references of screen helmets, many had gaps, now these are "Stunt" versions
Rubber S trim will take up some room, looks like you can remove a little more from under the front of the chin. One thing I can see though is heat bubbling, if it was me I would request another faceplate from the maker.
References from underneath
Your thread says "Hero" but they have a few differences to the standard "Stun" version, here are a few just on the helmet:
3 bumps on the ears, only one painted
Bubble lenses
6 teeth cut out
2 ear screws
Black inside sides of mic tips (not white like "Stunt")
NOTE no indents of outer teeth recess
More references here http://www.starwarshelmets.com/original-ANH-hero-Stormtrooper-armor-helmets.htm
Make sure you compare with the CRL text https://databank.501st.com/databank/Costuming:TK_anh_hero
With changing of a helmet and a few other pieces you can actually do 4 versions of Stormtrooper:
ANH Hero
ANH Stunt
ESB
Heavy Weapons
something to keep in mind for later.
Guide to differences between the OTTK versions
https://drive.google.com/file/d/10tEVteIreh6AHMogW010cuH_Bf8TBfU6/view