1976 DT400. Front shocks have been apart, cleaned, new seals, and I've been riding.I did find a stack of washers under the top nuts on top of the springs.I put them back in. What could it hurt? I'm not great at front fork rebuilding, don't really have a great understanding of how they work, get confused with different designs. I have a few different bikes. I don't notice poor performance from the shocks when riding, I'm slow, older and slower. I do notice a really bad fork dive when I get on the front brake, enough that I need to improve it.
I'm thinking progressive springs but do not know enough if this will fix the problem.

Does anybody know a seller of progressive front springs for a 76 DT400?
Any other recommendations?

I have also looked for answers to this question. I am watching for a reply. You could try thicker fork oil. Factory spec is 10w30 motor oil. If you are using modern fork oil this could be too thin. I am no expert I just start every repair with new, clean, and stock
specs and go from there. Not enough oil could also be the problem.  I  bought nylon rod from McMaster-Carr and cut 1 inch pieces and put them on top of the existing front spring spacers to achieve the same results. I thought it would be better than multiple metal washers and it fit more snug inside the tube with no metal to metal wear. 

Could try more oil & heavier grade.
There's probably some scope for adding oil before they hydraulic on compression rather than reach the tapered bit on the bottom of the damper that stops it bottoming out with a metal to metal contact.
Takes a lot to get them totally compressed while riding.
Forks 'do' dive though under hard braking [if you have an efficient brake].
Has it got the spacer on top [plus washers] or just washers in place of the split tube spacer.
Spacer should have a special washer under it on top of the spring to keep the spring central & stop it trying to climb into the spacer.
Plus ideally [& not original] another special same washer on top of the spacer just to save the cap nut & keep all tidy.
If someone's added washers including the spacer you need to be mindful of the springs coil binding before full compression.
www.partzilla.com/catalog/yamaha/motorcy...t-fork-250b-c-400b-c

RT325 made a great point about being mindful of coil bind... 

I will add that adding spacers (preload) has a minimal effect on bottoming resistance and can be detrimental to normal riding...  especially if forks become "topped out" due to the preload.

Increasing the spring rate is a far better method.  This can be accomplished very easily and effectively by simply shortening the spring by cutting it shorter and replacing the length with a solid spacer (PVC schedule 40 or 80 pipe makes a great spacer).  

This method does require care...  you need to make sure that the spring does not coil bind at full compression.  You need to look up the travel of the forks, add the amount of preload you're compressing them...  say the forks have 6 inches of travel and you're going to have a spacer length that compresses the spring 1 inch when the fork cap is installed...  that's a total of seven inches.  You'll need to make sure the air gaps between the coils of your uncompressed cut spring add up to more than seven inches... By doing some careful measuring you can get an idea of how much you can safely cut from the spring and/or how much preload you can use with the cut or even the stock spring.

Another option is new springs...  RaceTech might have something...  probably not though...  Cannon Racecraft in Oklahoma is awesome to work with and they will make you any spring you want.  I'm pretty sure both of these places have spring rate calculators to help figure out what rate you should order. 

www.cannonracecraft.com/