One cold day and you may find yourself wishing you had used one. Like everything else it's all about how much power you want to make and how fast you want to go.
The chances are your stock shaft is a used piece which was made in the 1960's and has tens of thousands of hours in service and possibly hundreds of millions of reveloutions under load. How that load was applied will make the difference between one which might fail and not fail. Do you know where your stock
oil pump shaft had been? I guess if you are assembling a stock 390 from parts you perviously owned it doesn't matter. If you grenade the motor it shouldn't cost you more than a few hundred to pull one out of an old truck and slap another $900 into the rebuild.
If on the other hand you have $15,000.+ invested in a .015" over side oiler and it will cost you $5,000 to repair the damage from a failed shaft then I'd recommend spending the extra $15. for a high strength unit from Ford Racing or other reputable source. Um, ok. On second thought I am sufficiently lazy to think the high strength unit is a good investment whether you are at the $1500. or $15,000. pricepoint.