Ah, that makes sense and first time I've heard of that. Coolant running down from the top of the core and out the bottom won't push out all the air, so it gurgles. It's not that hard to get the heater hoses swtiched on the lower and upper nipples on the firewall. Perhaps even easier if you're putting in new hoses. Some aftermarket inlet heater hoses come as one piece and you cut it to insert the heater valve, so when you size it for the cut you'd better be doing it to the lower nipple. If you'd reverse flushed the system it would have pushed out the air, but the heater would have run less efficiently. People used to reverse flush, installing a hose fitting in the upper heater hose (Prestone sometimes came with that fitting as a promo or you could buy it separately). I did that on my 140, but not since. Hose flushing leaves too much tap water in the system after the final drain, while de-ionized water or pre-mixed coolant is what should be used. I no longer flush unless I see a lot of crud in the original draining. Too much flushing can remove scale that is actualling lining the system and protecting the metal. Many people, including me, will experience waterpump and core failures within a few weeks or month of a coolant change. They were going to go eventiually, just maybe not so soon.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now