When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I just pulled my 300 inline six out of my 90 f150. I bought a engine stand at Advance auto that was rated at 750 lbs. When I hooked the stand up to the engine & started letting the hoists down, the bolts start bending. I also used grade 8 bolts.
It may not have been the weight that was causing your problem but the center of gravity for the engine. That motor is pretty long (and heavy) so the CG would be a good distance from the support as opposed to say a V configuration engine. I did several I-6 motor swaps in my '85 F150 and I was never comfortable when I had that motor on the stand.
As was mentioned, make sure all the bolts are good and tight and if necessary, support the front of the motor. Or get a stand rated for more weight.
The 750 lb advance stand is weak. When I had my carb'd 300 on one, it swayed and was very unstable. Its the physical length of the inline 6 that contributes most. When my complete 302 was hanging, the stand wasn't nearly a wobbly. The difference in weight between the two engines isnt that much. I say take the stand back and throw a fit that it doesnt handle its advertised capacity & then get a stronger one. No one of a sound mind will try to argue that a 300 weighs more than a 460 (700+lbs.)
That 750 is the one with one wheel up front .... I have seen them, the display models don't seem that sturdy. I'd go with the 1000LB one .... that is what the one I used for my lightweight Chevy Vega and 2.3L Ford motors years ago, and it held the 351C very well also, but I don't know if I'd put my 300 on it, the engine is a good 9 inches longer than my cleveland .... maybe more. Look at HarborFreight.com .... they have a 2000LB one for $100. It looks like it was put on steriods, has a wide stance out front, and it is collapsable.
Looks like the HF stand has more wheels and all swivel. The other looks like fixed in the rear. Makes it easier (and safer) if you need to roll it when loaded.
The little picture is from Harbor Freight too .... they didn't have one on the Advance Auto Site, but that is probably the one it was, it isn't wide enough at the back for a large engine and when you try to turn it it may want to fall over .... the 300 ford engine is just too much for it.