To the OP: I had your problem. I tried Hogue boots, full-size Hogue, current production Smith boots, etc. None of the grips really cleared speed loaders properly. There was always some sort of hang up.
I happened to trade for a '92 vintage model 60 that sported the ORIGINAL Uncle Mike's boot grips equipped on many Smiths at the time - the ones that were licensed copies of the Spegel design (current Smith factory grips are similar, but have dimensional differences that interfere with loaders a bit). Lo and behold, speed loaders work with no issues.
Purchased two sets of them last month direct from Smith's close out section of their online store. Looks like they've still got some in stock.
$14.95 a set is a great price - but shipping is a little high, so I bought two to offset the price - it's not like I won't find use for them!
Turns out I shoot better with them, too. Win-win!
Product: J Round Boot Grip Uncle Mike's (No Logo)
EDIT: Guess it wasn't too much for shipping. $6.50 to ship, charged me sales tax. So two pair delivered to IN for $38.49. For grips that work. Good deal.