Very ambitous PHIL82!
Engine swap, complete with Smartlock?
ABS install - means all new wheel-hubs/sensors/plumbing/electrics. Not easy, I hope you have a complete wreck/parts_car to be sourcing parts from.
Body separated? I assume you mean suspension sub-assemblies removed - there is no separate chassis to separate the body from. (Been reading too many Yank magazines? Hahaha)
If sticking with 15" wheels I assume you mean 235/60/15 & 255/60/15 tyres. These are getting very wide & tall for a Falcon. Are you sure the 235 this tall will clear at the top - may foul on the spindle/upper balljoint and or guard - especially when lowered - although 1.5" is a sensible amount.
I couldn't quickly find the calculator I was looking for but try this one;
About halfway down the page titled "A tire size calculator".
Compare your size with a standard say 205/65/15.
You will find that 255/60/15's are 6.1% taller, so to maintain the revs you have now will require a 6.1% shorter diff ratio (higher numerically). Also note that actual tyre sizes may differ slightly from the theoretical calculations.
What do EA have as std diffs 3.08? So a change to around 3.27 would maintain current revs with 255/60's. A 3.45 will have it revving a bit harder for a given speed. It's the tyres circumference (rolling radius actually) as well as the ratio that determines revs per km/h. I don't think there is any legal limit on diff ratios, it's more about what is available and sensible/usable.
I like your colour ideas.
I would go with say 235/45/17 & 255/40/18 on 17x8 & 18x9 myself - Simmons make these sizes but pricey.
Why not the 4-speed auto. Nice box and the overdrive would help with 3.45's
I hope that all gives you something to think about.