The change from 220° @ .050" to 250° @ .050" is huge, and will have considerably worse "manners", including higher minimum cruising RPM in O/D.
Even though the duration is similar to the E2, I would guess that the LSA is tighter and the installed ICL advanced slightly. Idle quality will suffer; this may improve by adding 2-4° to the initial spark lead (an subtracting it from the total by shortening the curve).
In my opinion, it's going to increase peak power, but raise the torque range, meaning that the added power will require higher RPM before shifting.
AFAIK there are no aftermarket rockers at all for the GMC, and not many that can be adapted because they're so long. I suspect some big truck rocker may be suitable, but no actual choices so far.
What's the exhaust like?
