re jetting carb

[djspn]

I've been running a PO installed 200 cfm Holley Bugspray, w heat riser, extractor system and 009, on my otherwise stock '71 bus for over 20 years, am happy w it.
Was getting 20 mpg w old engine, better than that w new Mexi longblock.
Am going to rebuild and probably rebush (or anyone know of any NOS?). Believe I also need to lean out the main jets while it's off as it seems to be running rich, perhaps because I replaced extractor w stock exhaust.
I've recently made a trip from Minnesota to California w a few 9000' passes along the way. At higher altitudes there was a noticable lack of full throttle power when climbing in 3rd and 4th. Now that I'm on the coast it's much better.
I don't know much about it, but seems to me to indicate that main jets are too big???
What else can I look for to confirm this idea that jetting is too rich???
Plugs looked OK but only had 1500 mi on them.
Timing is set 7.5 btdc static, ~32 full advance.
Any suggestions re size increments when changing main jet???
Thanks
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Peace
'71 Bus .....stock running gear [ex SVDA and pertronix]