On the Gen 6 BBC, the block drains are plain pipe-thread plugs, one on each side of the block. They are not like the SBC which uses a knock sensor on the right side, threaded into the block drain hole.
I'm not so sure about the TBI BBC--Gen 5, or Mark IV. They may have block drain plugs, or they may have a knock sensor screwed into one or both block drain holes.
At any rate, they're just plain ol' 1/4" pipe thread holes, which means you'd want either the knock sensor, or a plain ol' pipe-thread brass plug or draincock depending on what you removed.
Available at any local "home improvement" store, or auto-parts store. If you have to put the knock sensor back into the drain hole,
be sure to seal the threads and torque to spec.
I have photos of the draincocks installed on my '97 L29 7.4L. I use Loctite/Permatex "592" sealer on all pipe-thread fittings.
Right side, ahead of starter motor, below knock sensor. 45 degree street elbow in the block, draincock screwed-into the elbow. Had to grind the elbow some to get it to clear the dipstick tube as it screwed in. The angle of the photo makes it look like the draincock points "up", but that's an optical illusion. It sticks out of the block approximately level.
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Left side. Draincock screwed directly into block near oil filter and knock sensor.
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Drain the old coolant into a drain pan, dispose of according to your local regulations. Some places want the old coolant to be "recycled", other places allow you to dump it in a toilet so it goes to the local water-treatment plant. DO NOT dump it on the ground, or into a storm sewer.
I would use a Flush 'n' Fill kit, inexpensive and makes the job easier.
www.amazon.com/dp/B000CCFY5W/?coliid=I2S26W9IAZJJ30&colid=2VLYZKC3HBBDO&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it
Similarly, I've also been using the Lisle bigass "no spill" funnels for refilling the cooling system. Beware of cheap-junk knockoffs.
www.amazon.com/Lisle-24680-Spill-Free-Funnel/dp/B00A6AS6LY/ref=sr_1_1?
While the coolant is drained, you have a perfect opportunity to replace the upper and lower radiator hoses, the heater hoses, and install a block heater. Consider replacing the water pump, thermostat, and heater supply hose quick-connect fitting, which are often rotted-out. The Dorman replacement quick-connect is actually MUCH better than the Genuine GM item.
Inspect the radiator for internal (water passage) problems such as scale or leaks. Inspect for external problems such as folded-over air fins, or corrosion on the air-fins from salted-winter-streets road splash kicked-up by the car in front.
The coolant overflow bottle is often filthy. I've cleaned 'em out with ordinary playground sand, hot water and some detergent--laundry preferred, dish detergent acceptable. Remove the bottle and hose from the vehicle, and plug the hose . Add a cup of HOT water, some detergent, and a cup of sand to the bottle, shake it like you mean it. Let the sand scrub the scuzz from the bottle. Drain, and repeat if needed.
Use the owner's manual or service manual to determine cooling system capacity. Buy CONCENTRATED anti-freeze, not the damned "premixed" "50/50" anti-freeze. Put the right amount of anti-freeze in the vehicle FIRST, then top-off with water, saving a little anti-freeze for the coolant overflow bottle.
