Missing at cruising speed. Confused.

[YourBuddy'sTruck]

I posted a while back about this, but it was way to cold to work on the truck.
I've been experiencing missing, but only while cruising at 50MPH or so.
No trouble at 35MPH or while accelerating.

It started to backfire out of the carb on first start of the day. (scares the begeebuz outta my Wife, too!)
However, after that first start, it doesn't backfire at all.

So I started thinking Timing Chain?

I switched out the plug wires, thinkin that the originals couldn't handle the new HEI.
Then I got to thinking...yeah, I know...there I go thinking again...
Could I have the original Leaded Gas Heads?
I've only put about 2500 miles on the truck since I got'er runnin' last year.
And I only drive about 15-30 miles in a week doing deliveries with it.
Could it be that it took 2500 miles for the problem to show up?
I put in some Instead-O-Lead and it is greatly improved! Only missed once on me in the past 50+ miles.

So my questions are....
Can you tell a Leaded Head from the outside? Or do you have to pull it off and look at the valve seats?
If so, did I damage my engine? If not, am I damaging it with the Instead-O-Lead?
I'm just old enough to recall the Green and Red gas, but not to know what the symptoms were when you mixed'em up.

My truck is a '70 C2 with a 350ci and a TH350.
The engine block number is correct for the era, but I doubt it to be original, then again? It is 3970010

I bought it with a completely empty cab and obviously it was not "running".
Spent the spring getting the puzzle back together with the help of many threads here.
Don't really know much more about it other than it had a seriously bent steering linkage and other front end parts that had the wheel at 90* to go straight.
Like someone hit a ditch or something with it.

Anyway...what are the symptoms of Unleaded in a Leaded engine and vice versa?
Thanks for any help.