I couldn't imagine being unable to get the block drains out with an impact. What type of engine? Usually we're talking brass NPT plugs with 9/16" hex heads. I've always had them come out relatively easy with a long handled flex head 3/8" drive ratchet, even on boats that I know they weren't regularly removed for winterization.

I drained mine last weekend, a 454. The block plugs came out like butter. Watched and they drained probably several gallons, with minimal debris, and ensured holes were not plugged with debris. Then pulled the uber NPT plugs in the back of the manifolds. I doubt they're been out recently. I hit them with PB and heat cycled them once, then used a cheater bar on my ratchet since I couldn't get a breaker bar in there. Took some grunting and groaning, but got them out and both manifolds drained with only minor debris coming out. Also pulled a plug on my raw water feed line between the raw water pump and trans cooler.

I suppose with those 5 plugs pulled and drained I shouldn't worry about it, but this weekend I'm going to pull the thermostat and run a few gallons of RV antifreeze through it, then pull plugs again to drain that. I worry about small amounts of water anywhere, particularly in the raw water pump or trans cooler that could freeze and do damage. I'm probably paranoid and it would be fine as-is, but I'd rather be safe than sorry.