Without knowing your intended application, it is impossible to comment.
However, the girl at Optima either made an error or has no idea what she
is talking about.
Their is absolutely no problem with fittment using the stock tray, I can
post pictures to show that. There is however a significant difference
between a red top and a yellow top battery which she should have mentioned.
While both use sprialcell technology, they are chemically different. The
red top is a starter battery while the yellow top is considered a deep
cycle battery.
Generally, the yellow top batteries are more desirable for deep cycle
applications whereas consistent performance is required while the battery
is discharging heavily. The most obvious applications being higher
performance car stereos, hydraulics, and winches.
You can view the technical specifications, for the D34 yellow top here:
http://www.optimabatteries.com/products/pdf/D34.pdf
There is an exactly 5lb difference between a D34 yellow top and a 34 red
top. While the D34 has a slightly lower CCA rating (750 vs 800); its open
circuit voltage is much higher (13.2 volts vs. 12.8) due to the lower
internal resistance.
So again, it really depends on your intended application. Batteries are
all about compromises with size/weight being directly related to reserve
capacity & CCA.
IMO, if your planning on parking any sort of higher current amplifier or
operating numerous electrical accessories for long periods while parked
(e.g. laptops, game consoles, etc...) I would reccomend one of the yellow
top's over a red top.