As a suggestion - get the largest can you can fit - on a short header - the JMB is still tops in my book as it is priced well has really good sound supression and works best on shortest possible header . the wrap around header also works - the shortest one is a bit better . no big deal.
The large Supersonic muffler for Super 80 (I guess you have one?) also works much better than the smaller in cowl types - noiser but more poop. make an adapter plate and try it on your new engine .
A tuned can - different ball game - these can work well or bite you in the butt. they are simply a tuned device which need to be properly set in length for the rpm band you are after -like a pipe .
No reason you should not get power readings similiar to what I get -if you use same stuff- set same way . no secrets here .
Tho our altitude is higher - all in all the same stuff reads the same way - -actually I get less power -tho the air load is less on th prop it pretty much balances out - read up on snowmobile etc., setups where they try to get more power at altitude - these engines suffer same losses as any IC .
I had a clown on another venue try to explain how readings at higher altitude were not -could not be accurate -
there are volumes of info which show how the prop losses ,engines losses relate - the latest chart Pe Reivers makes available is a good one to poke in numbers and se vas ist!