Even at the slight bump of the key mine will spin down for 4-5 seconds. I went on youtube and found some videos of others starting their cars and indeed some do stop almost instantly when the key is let off.
check out these videos
Reduction gear starter on a 1st gen hemi - YouTube
20-degree Slant Six cold start - YouTube
As far as getting a starter specifically for the six,I have always heard they are non-specific and can be interchanged among the lot,but there was never a specific starter for the slant six only. How can I identifity the "six banger" one?
heres a video showing the short and large frame starters,he claims they are all interchangeable,and the large frame didn't come out until 1975 for use on the bigger v8s to combat heat related hard start issues. until then he says that Chrysler used the same starter for the slants and V8's.
Identifying Chrysler gear-reduction starters - YouTube
in this video he tests them and they all seem to stop in seconds.
Chrysler starter comparison - YouTube