Does anyone know if these shifters...


...can be mounted onto this or any other down tube clamp?

That looks like a non-starter sorry to say.

I've put Shimano DT levers onto a Huret clamp with excellent results, even though I had to use bolts with wrong threading (plus LocTite) to secure it.

The picture below doesn't show things very well but here you go.

I bought a set of 6400 6/7 speed shifters on a Shimano bar clamp. I don't think they'll fit on the Suntour clamp- and I don't think the Suntour shifters will fit on the DT bosses.

A very big maybe. Shimano introduced their own design of braze-on sometime in the early or mid 70's (don't quote me on it). It was featured on most of their clamp on shifters and is commonly known as "type A". This is what the curved metal adaptor is adapting to. If you figure out some way to shrink the adaptor down, you can fit the index shifters onto clamp-on shifter bosses. Otherwise, they work on all braze-on shifter bosses.

I'd say it's a maybe. Your Shimano levers are later production, by the time they went to SIS/indexed they were spec'd to mount on standard dt bosses, not the earlier Shimano-only "A" type.

The problem is I don't remember what lives under the Suntour Power Shifters, whether there's some kind of shoulder build into the clamp for the shifter. If not, and removing the lever just leaves you a flat surface with a standard boss-type mount, you're approaching good.

But only approaching good because your Shimano levers use a curved/radiused shift lever boss cover, the silver piece that goes between the lever and the frame, but what you'll need are flat ones to fit the clamp. You can buy 'em, maybe $12-$16/pr? They're usually stock on Shimano bar-cons.

So it's a kinda/maybe/who-knows?

I recall the suntour lever clamp was assembled in a similar manner to how Campagnolo did it
the backing plate is trapped by a crude rivet made up of the boss, inclusive washer shaped region and internal threading.

on the Calpagnolo levers we used a countersink drill off the rivet splay and harvest the backing plates mount those onto the brazed on boss and use a different Campagnolo thick washer - first steel later plastic.

worth taking a pair of the clamp on ratchet levers apart and reference dimensions.
i think something will need to be fabricated to make it all play

I just realized that I do not need a Suntour clamp but I do need a Shimano one. If I can find a Shimano clamp on shifter set, do you think that would accept the shifters shown?

Must have had two glasses of "stupid juice'" the morning I started this thread. Anyway, any advice would be a help. I don't want to buy a clamp if it will not work. I can always turn the bike into a junk single speed but I did have other ideas