It's when you grab the wrong gear in a down shift that'll rock your world (say out of 5th and into 2nd...).
In your case, you're pretty much protected by the rev limiter. You may want to have the vavles adjusted to make sure things haven't become too tight or two loose, but otherwise you're probably in the clear.
There's a reason Road & Track had referred to the 78-83 SCs as "the most durable car know to man".
If you had a bent valve, you'd definately notice a decrease in performace... or a car that "fails to proceed" as Rolls-Royce folk say.
Chalk it up to experinece and but that cruise in the crap can.
Happy driving!
