Check the heads for cracks.
You'll likely find some small short cracks on a few cylinders between the valves, these are not bad.
The cracks that are bad are those that start at a valve and continue accross the combustion surface of the head.
Some gaskets are harder to detect failure. What kind of gasket was on it? There is a gasket that's made of a softer metal like material, that you get you scrape off (but easily) off the deck and head. The other is a plastic/paper type gasket that usually comes off whole and retains it's form.
Anyways, look for any rings on the cylinders on the gasket that could have dislodged itself from the surrounding material. The usual suspects are on the ends of the gaskets. There is a hole in the gasket for coolant, it has a small amount of material separating that hole from the ring on the gasket. This material moves, and then the ring moves, and you got a blown gasket.
These spots will be beside cylinders 1,2,7,8
Check for pitting on the deck surface in these areas as well.
The temperature swings are a sign, as well as the coolant loss. I do doubt that you've got a cracked block or coolant mixing with oil, the smoke was likely blowby.
The cracking block thing is more common on '97+ engines where cracks get into the cylinder walls leading to water consumption. But, it can happen, just differently.
good luck.
1982 C10 SCSWB 6.5TD, mods too extensive to list. (13.69 1/4 mile @94.6 MPH) RACE TRUCK
1982 C10 SCSWB 6.2NA, 2.73 700
1986 C10 SCLWB 6.2TD 3.73 700
1989 V20 SUB 6.2NA, 3.73 400
1994 G20 VAN 6.5NA, 3.42 60E
1994 K20LD ECSWB 6.5TD, 3.42 80E
1995 K20 SUB 6.5TD, Wrecked, ran into by stupid teen.
1995 C3500HD DRW 6.5TD, 12' Flatbed 5.13 80E
1995 C3500HD DRW 6.5TD, 18' Rollback Wrecker 4.63 80E
1994 C20HD ECLWB 6.5TD 3.73 80E Wifes Truck.
1995 C20LD ECSWB 6.5TD 3.73 80E
1995 K20LD SCLWB 6.5TD 3.73 80E
1996 K30 DRW 6.5TD 4.10 80E
1997 C10 Tahoe 2Door 2WD 5.7L to 6.5 Conversion Underway