Mouting an old Cannondale on a smart trainer
 

So I've got an old Cannondale (SR400 or similar – have never been able to find it in the catalogs) with a 12 speed SIS drivetrain (Shimano 105, Biopace crank, etc.). I've got a Saris H3 smart trainer that I've been using with my one 'modern era' bike. Wondering if I can repurpose this Cannondale as my trainer stand bike? Don't want to make any permanent alterations, but it's not intended to be a museum piece either. Wondering if the age of this bike will make it more trouble than it's worth. (Meaning, I should just go out and find a more modern beater style bike to mount on my trainer.) Any advice welcome.

Rear Dropout Clearance.
So my Cannondale measures 126.5mm. Saris minimum rear dropout clearance is 130mm. After researching the forums here, I've come away with the conclusion that the Cannondale frame is made to adapt to 130mm. So no issues here.

Freehub vs. Cassette
The Cannondale has a 6 speed, thread on style Shimano 600 freehub. Saris specs require an 8-11 speed rear cassette. The Saris is a wheel off trainer, and I use it in ERG mode almost exclusively, so I don't even really care about shifting. I just need to find a modern style cassette that can work in the frame. Is there such a thing as a 6-speed cassette? Assuming I could account for any spacing gaps with rear cassette spacers?

I use an old crit C’dale on a yahoo trainer. It is 126 spacing spread to fit the trainer. Mine is running 8 speed friction but I see no reason why 6 speed would not work with spacers. To be honest spreading the rear to fit kind of makes me cringe but It’s been mounted there for several years without issues.

So is there such a thing as a 6-speed Shimano compatible rear cassette (not thread-on freehub)?

You should be fine, as long as you space the cassette to fit a 35m freehub. Since you run it in ERG mode, am assuming you run the same gear all the time?

Yes, because of ERG mode, I don't really need any shifting ability at all. (The trainer does that for me.)

There are about about a dozen different options. Move to 7 or 8 speed drive train.
7 speed cassette and lock off a cog. Etc.

Yes, there were 6 speed cassette cogs back in the 80's but they were for uniglide freehub body, (without the one different engagement tooth that hyperglide cassettes use to properly align the shift ramps).

Best option is to get a hyperglide in whatever speed would match the chain of choice and add spacers behind the cassette.