Your car may be suffering from something people often did but were not aware of - replacing a four cylinder with a six cylinder when the car originally didn't have overdrive. A six cylinder Henry J without overdrive should have a 41/10 (4.10) rearend. You can verify it by looking at the tag on the rear end (it is on one of the cover bolts). A Henry J six without OD will say "41/10" on the tag. If it says "41/9" on the tag, then the car was likely a four cylinder that someone installed a six in and was not aware that the rearend needed to be changed as well.
One thing you can do, if the car has the 41/9, (other than finding a 41/10 rearend for it) is add overdrive. There's one for sale on ebay right now and the price is not too bad -
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=250663044295&viewitem=&sspagename=STRK%3AMEWAX%3AITThis seller has been parting out a Henry J and would likely have the control cable, relay, kickdown switch and connector as well. If not, you can buy a generic cable or if you can find one for a 1949-50 Kaiser or 1949-51 Frazer, it's the same cable.
The drive shaft will have to be shortened too but any machine shop can do that... or perhaps the seller of the trans will sell the driveshaft too.
In any case, adding overdrive to the car will allow you to cruise right along at 65 - 70 mph with no problems. I do it all the time.
The numbers on your car will also tell the tale - if the model number on the VIN or body tag is K513x then it was originally a four without OD and someone installed a six. If the tags list the model as K514x then it was originally a deluxe and probably not ordered with OD (which was not uncommon). I have friend here with a six without OD and he drives it all the time and says he can do 65 mph pretty easily and his has the 41/10 so my guess is the car you're refering to is a mismatch. There was a very nice '51 for sale on ebay last year and I know for a fact that one was a mismatch. I don't know whether the seller knew or if he informed the buyer. The car was in Iowa I believe.