Hi Luke. Did not understand your question/ problem at first, but upon contemplating some reasoning for a
while I might share some things here.
You don't say what v8 and which auto transmission you have there.
Is it carburetted? Do you have rev hang/ match through vacuum dashpot?
Is your rev drop instantly on throttle release, or does it decay slowly? What is the general throttle response?
Do you have non-linear throttle control? Is the autobox lockup? Is it ecm or pressure valve controlled?
What is the gear ratios? etc, etc, etc..
You see..there's only one thing that might cause push from the engine, and that's by gooing the juice.
Other than that, upon throttle release, engine braking will occur from the wheels, through the drivetrain,
auto box and TC...dependent on alot of variables that comes into play here.
Second aspect you need to understand is that there is unfortunately nothing you can do about the physical
weight of the vehicle...unless you leave the cleanmother and baggage at home.
As far as the tuna is concerned, I don't know which one you drove, but on the 1GR-FE it is very, very low
geared that upon throttle release will not have a substantial adverse or opposing affect unless you stick
it into 3 or 2L...and that is exactly what the manual recommends.
I'm not currently aware of any auto trannys out there that mechanically disengages the TC turbine from the
output shaft or shifts the a/box into N or any of those solutions upon throttle release and braking.
I do however understand that the TC lockup does release under these circumstances.
So it basically comes down to autobox properties, TC dynamics and gear ratios. Which in good harmony with
whats under the bonnet and in good balance will give you the wanted effect.
So not an easy thing to achieve here.
Hope it helps.