WAS kit is good, no doubt. I use a fair few bits myself, BUT you are in the league's of the big boys price wise and you can pick up eagle industries, HSGI, emdom and tactical tailor kit for similar prices which are arguably slightly better quality.
https://www.ar15.com/forums/armory/M16A3__is_it_a_flat_top_like_the_A4_/23-271767/
I stand corrected, it appears AR15 A3's from colt are flat top.
M16A3's appear to be a very rare beast indeed.
I've bought and sold literally hundreds of guns over the years. As others mention above i rarely used to lose money when reselling, i reckon there's a time most aeg's in use were 3rd/4th hand.
But selling these days seems slow.
On the other hand,
People seem to mess with/ butcher/ upgrade...
What I'm trying to say (badly) is they all work in a similar way. The holster is attached to the platform by a bolt passing through a hole in the platform, as long as the head of the bolt is wider than the hole in the platform it should work with just the bolt from the holster. Even if its too...
Screw size won't matter, just use the screw from the holster on the new platform.
Ive used fobus roto platforms with IMI holsters with success but no direct experience of nuprol.
Ferro danger pouch attached to the bottom of the carrier, covers the wobbly bits for me.
Problem is, your plate carrier is actually covering the bits it should and is designed to cover Heart, lungs, liver etc.
You may be better off with a more generic armour type, like an osprey or IBA etc. ...
https://www.uktactical.com/p-3704-imi-rotating-molle-attachment-black.aspx?gclid=EAIaIQobChMIhfaMoLzU3QIVCLvtCh2cbQq_EAQYASABEgJm-PD_BwE
Something like this, or
https://www.amazon.co.uk/Fobus-Molle-Platform-Mounting-Rotating/dp/B079BX6S4T
This
Should work.