I only made the suggestion that I had tried it and it worked for me.

If you change just the 21 to a 28 you will have made something like the mega-range cassette with a big jump to the one and only granny gear rather than a range of low gears without a big jump. Both methods will give you a low gear of 40 GI aprox.

If you go with a new cassette 12-28 you will have wider spacing between gears on the small ring.

Keeping the 52t ring it will still be one you don’t use with a top gear of 115 GI.

I agree it doesn’t look classic but you can keep your old crank, BB if required and chain and be back to your classic to resell with 15 minutes of work and no wear on the original parts.

I had to lower my FD and it worked fine not shown in photo. Also the chain line of the triple put the chain in a good place for me liking the smaller ring and the big ring lines up with the small cogs.

It was just an idea I had and it worked out for me. I had planned on putting it all back after I tried it out on a rather hilly ride I had coming up but liked the lower gears enough I left it with the mtn crank. And still have the old components in a zip lock bag.

Here is what mine looks like. Not pretty but works well.