Unfortunately even though conversion kits are the greenest way to transition to electric, I don't see them growing much.
Good kits + the labor to install them typically cost more than an entire new replacement vehicle with all the modern bells and whistles, and they can't be warrantied and serviced in a straightforward accessible way.
The economics of the situation make it so it's a better choice to either drive the vehicle as an ICE car until the end of its life or just replace it with a brand new electric car.
I expect conversions to stay extremely niche for this reason.
The Grumman LLV's already have 30 year lifespans. There's not a huge reason to extend their lives further at this point, and more importantly the needs of mail carriers have changed. The LLV was designed and sized for mostly envelope mail and minimal packages, but the internet, amazon, and covid have both changed the game and now packages are a huge part of the delivery work. Those LLV's are just not up to the task or the volume, which is part of why they've already replaced so many of them with modern vans.
It doesn't help that it doesn't have air conditioning, and the fusebox is right under the wipers which causes alarmingly frequent fires.