Just a note....if you have the tangs welded, it may have an effect on the casehardening of the same? I saw one gun that had been nicely welded and didn't show at all, but the weld area cracked when cased.
Might want to go with rust blue after welding. It wont show, and strength wont be compromised. John Taylor at: john@johntaylormachine.com  can do the welding, if you go with the rebuild.

Paul, depending on what you want to do with it, you might get lucky and find a lower tang (original) that could be bought cheaper than a new one...if you are going to refinish it anyway.  Which low-wall coil spring action is this, the smooth side (no flares at front and back that some mistakenly call a thick side) or the larger flared type that was used for the 3rd Winder Musket?  If the latter, you can use any high-wall parts you like except for the breech block, if the former, you are limited to the smaller smooth side low-wall parts. If you want to buy the tang new, or any of the parts for the trigger assembly itself are probably still available from Ballard Rifle.  Also the casting kits for DST are (or were) available from whoever is handling the Rodney Storey castings.  

Is the top tang bent, shortened, or unaltered?  Since that is part of the receiver itself, you will be forced to make that right or somehow allow for it upon restocking.  I have had one high-wall with shortened tang built up and reshaped, but I have not tried to recase it yet and am thinking maybe it will be blued instead.  You are right about the filler material, but it seems to me that strange things happen anyway when welding has happened.  I still regard it as something of a black art rather than a science!  

All for now, Paul but I hope this has been of some help.  Keep us posted on your progress, maybe it will encourage me at least to get some more done on MY projects!  

Froggie

Paul,
If the pistol grip fits - wear it.  Why change?  The top tang is only critical if you want a tang sight.  If you do, it is probably easier to find or make some sort of base to fit.   

As for the single set, I think they are voodoo myself.  Check with Joe Harz and buy some parts from Ballard Rifle company.   

You will not make a modified gun into an original, so you might as well make it into what works best for you.  At least, this was my approach and I can't be happier with my low wall.

Brent

Paul_F -----   for best results regarding welding & re-color case;

1)  have frame annealed by Classic Guns ($25.)
2)  have all welding/bending done now. By annealing first, you will get a better weld and the new color case will take better. Also, you will find it is much easier to hand polish. Stress to welder that he is to use only LOW CARBON steel wire !  By doing it this way, you won't have any tell tale areas where the weld has been done.
3)  hand polish all areas of action & lever to a 320 grit "wet or dry" finish and fit stocks.
4) after wet sanding stock to fill pores, begin final finish.
5)  finish polishing action & lever to min. 400 grit or 600 grit as I do & send to Classic for color case.

Good luck,   Ken

Paul, IMO your SST should present very little problem & is worth pursuing. PM me for more, I'll be happy to help.

However, the fact that your upper tang has been shortened to such a great degree makes me think that it would be better to either use it as is or sell/trade it for an unaltered one. If you do decide to weld it, make sure to use very low-carbon steel, not only for the rod but also for any new section added to lengthen it. IOW if it's now an inch short, the usual procedure would be to weld a 1"-long piece of steel to the end, and shape it. In this case you'll have the added piece and the welding filler rod. both of which must be low-carbon to match the original. If either is high-carbon, then that portion will be too hard when cased and will crack. Also, high-carbon steel always requires a pre-heat and a post-weld heat treatment (PWHT)(basically a slow cool-down) to avoid over-hardening and cracking. Altogether it's a hassle.
Good luck & PM me, Joe