The original Down pipes should be double skinned.If at all possible it's better to have these repaired and re chromed as the after market ones,unless stainless steel,are crap.If you mixture goes lean with these single skinned AM ones you will get Bluing/Golding of the pipes very quick and then that's a pain to get rid of and they corrode quickly.
Post above is a good idea if possible e.g get some Stainless made up by an Exhaust specialist.
FYI there are Stainless H-boxes available for a price over here in the UK.
Of course there's always a Motad 2 into 1 as well,
I've had a couple of Motads through my hands and they are extremely good quality.They also have the advantage over the Mac exhausts inasmuch as you don't need to re-jet the carbs.They are fire-and-forget and are supposed to add a few BHP to the bikes.
I've also had direct dealings with Motad and they were pleasant an efficient.I passed on the exhausts as they were little too quiet for my liking at the time but my tastes have now changed again and I kind of regret not keeping at least one set of them
They are much easier to have on and fit to bikes than the normal down-pipe/H-box set-up.
Note:They do get hotter than a twin pipe as you would expect as the exhaust gases are going out through a single chamber.
I found that new pipes from David Silver are much cheaper than chroming. Apparently, the environmental regulations re chrome are now so cost-prohibitive that only the most necessary jobs are worthwhile. Please correct me if I'm wrong. I think my new (chrome) pipes cost me about 80.00.
The can be re-chromed, and most fo the cost is removing the old chrome (you can NOT chrome over chrome). Chrome is getting very expensive as the chemicals are nasty and the regs for there use get stiffer. There are alternatives out there that are just as good and far more cost effective. Casswell make a "Khrome" coating that looks almost identical to real chrome (one has a slight yellow ting, the other a slight blue, cant remember which) AND is 4 time tougher. Its nickle based AND uses a Non-electrolytic system, just dip and time.