Just to expand on my earlier comment on different sized orifices on the venturi cluster-
Yes, there are different size jets on 421 vs 389 intended center carbs, but the cluster itself also has different orifice outputs.
I am not sure if there is a tripower sorta of addendum to the standard chassis shop manual, I doubt it. My '65 Chassis manual has all the information right in its carburetor section. HOWEVER- I did get a Rochester 2bbl rebuild guide booklet off of ebay. People sell reprints of these all over ebay for pretty cheap and it might be worth your while as it expands a little more on the ins and outs of the the 2 barrel carbs. I do believe it has info on end carbs as well as center carbs. It's a grey-covered manual.. i did a search for something like it on ebay but didn't have any luck. Here's one for sale elsewhere:
http://carburetor-manual.com/rochest...968-p-150.html
These are all helpful but none have had ALL the information that I've obtained from other real-deal guys who have worked with these things extensive.
A decent thread to browse at is my extensive thread on my rebuild:
http://forums.maxperformanceinc.com/...light=tripower
Sadly, it's mostly on the restoration of the one I used, and I said, never got to the install / tune of it.. but im sure there are useful tidbits in there.
Do a search on here for like "tripower install problems" or something like that and see how other people diagnosed + fixed their problems. I've done such a search before and I know there were some good posts.
keep us posted!