My 1985 impala 4 3 v6 is backfiring when accelorating

I doubt the problem is a head gasket.
It sounds like a wiring problem or something affecting the injectors and/or ignition. A very lean fuel mixture could cause the running rough and backfiring problems. So could an ignition problem where the plugs are firing on the wrong cylinders.
The V6 engine does use plug wires, and its possible some of the plug wires are on the wrong plugs.
A leaking intake gasket could also cause the symptoms.

For the 2000 Chevrolet Impala 3.8L V6:

Firing Order:
1-6-5-4-3-2

firing order for 2000 impala Google Search

The most likely cause of that problem would be the ignition distributor assembly. Inspect it and replace with new parts if you see any wear or carbon deposits. They are usually pretty inexpensive. Check the engine timing. If that all sounds difficult you could try just loosening the distributor and making minor adjustments in the direction that seems to help.

Sound like the engine jumped time.

V6-262 4.3 VIN 7 TBI
V6-262 4.3 VIN Z TBI
V8-305 5.0L
V8-350 5.7L
V8-350 5.7L DSL ?????????????????????????

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For the 1985 Chevrolet S-10 Blazer:
I do not know if you have the 2.5L L4 or the 2.8L V6, however I have included both below....

For the 2.5L In-Line 4-Cylinder:
Firing Order:
1 - 3 - 4 - 2
Cylinders:
(back)
'..4..'
'..3..'
'..2..'
'..1..'
(front)
Distributor:
...(back)
.....(+) - screw
2.........1
4.........3
.....(+) - screw

For the 2.8L V6:
Firing Order:
1 - 2 - 3 - 4 - 5 - 6
Cylinders:
....(back)....
'.5..........6.'
'.3..........4.'
'.1..........2.'
....(front)
Distributor:
.....(back)
.....3....4.(+) - screw
..2...........5
(+).1....6
.....(front)
Let me know if this helped......

For the 2001 Chevrolet Impala
I do not know if you have the 3.4L or 3.8L however I have included both below:

For the 3.4L V6:
Firing Order:
1-2-3-4-5-6
Ignition Connections:
............(back)...........
.'.5..2.'..'.3..6.'..'.4..1.'
............(front)

For the 3.8L V6:
Firing Order:
1-6-5-4-3-2
Cylinders:
.....(back).....
'...2...4...6...'
___________
'...1...3...5...'
.....(front)

Ignition Coil Connections:
(back)
o 6
o ---3
o 2
o ---5
o 4
o ---1
(front)