1. 60mm, and available sizes are also 65mm and 70mm.
You can get any of these bore up to 2 or 3mm oversized
and the true aftermarket ones maybe up to +5mm.
2. I have done the 70mm without welding the flange, but above that
is probably goning to need welded, and most shops have to weld to
get the 70 to fit, but I use a special trick
3. For most 3.8's, I recommend 60-65mm but anything over the
stock 60 really needs lots of porting to the crossover tube if not
post removal. You might be able to use a bored 60 (62-63) if the
crossover is properly ported but has the posts intact. But that is
the Max, according to the cross sectional airflow calculations and
the "cheat factor".
4. None. If it helps on any engine, and I doubt it does, this one has a
specially designed crossover that in my opinion would actually benefit
from less swirl instead of more due to the shape and design of it.
But that is just my opinion, and I can actually mod a TB to "swirl" the
air probably as effectively as those spacers if not more so. Most of them
actually have a design that looks restrictive rather than swirl producing
