Head Gasket ?

93 GMC Jimmy 4.3 W

After a 400 mi drive I noticed I had no coolant, after hearing the truck sputter at idle and low speeds. I filled it up, let it cool down and drove another 10 mi to a garage.

Engine oil - NOT HIGH, I actually put a 1/2 liter in. I don't recall the oil being milkshaky as someone suggested.
Coolant reservoir was bone dry.

I have replaced CPI, EGR, within past year. CPI was from ebay but it has run well.

One plug had nasty carbon deposits (or sim.) and the compression was only 30psi. Other plugs measured at 120+psi. Garage told me it was prob the cylinder and I'd be better putting a new engine then paying them to repair.

My trusty Haynes manual suggested putting oil in the plug hole and retesting the compression to figure whether it is the head gasket or the rings. The conclusion was that it was the head gasket.

I am about to try to do this myself w/ a couple cycle mechanic friends. Wondering if there are any last minute suggestions for me, regarding having faith in this garages twice-revised diagnosis, regarding other things to do while I'm inthe engine.

What size tap must I have to clean the bolt holes in the block? Or could this be avoided since it's a factory engine?

ANy advice will be helpful. I can usualy find answers on the archive but I am shy on time due to a death in the family.

thanks

gregory