Since you've asked for How to vac secondary carb your tri power, you've got the best info from us all. Go have some fun.
As for me, I'd not personally ever use a vac sec carb, as its a well defined "air on demand" carb, verses the far more resposive mec sec, or independent runner or harmonically balanced 3rd, 4th and 5 th order tunnel ram. The latter ones give better results.
See
http://forums.corvetteforum.com/c3-tech ... aries.html regards just the debate between vac sec and mec sec
The ideal vac sec is the 630 cfm vac sec 4180 Holley 4-bbl, then the 3514V Australian GTHO/Chevy Big block 396 750/780 cfm 3310. If you emulate the heavily dampened secondary, you'll always do well economy wise, with a nice,crisp mid range. For vac secondary 6-bbls, the Six Pack Mopar were okay when the secondary circuit was used but a nightamre when not. The Olds J2 were better, as the carb never suffered the problems the Holleys did with drying out.
(Do note, that as I've said before, GM out engineered everyone with there mechanical secondary Qjet and Rochester Mechancial outers on the Pontiacs. The 389 GTO and 421 Catalina were the best Tri Powers ever, especially mid range. Vac operation doesn't ever yield better performance, only lighter accelerator pedal and it covers fuel economy and adjustment simplicities, and later, the ability to eliminate the outers during the emission cycle, especialy true of the Mopar six packs. Adding a spring to the vac dashpot allows the level of severity to be tuned.
Its no different to a mech sec 4-bbl verses vac sec 4-bbl, mid range is stouter with up to 10% more torque with a mec sec. Fuel economy almost always takes a dive when mechanical secondaries are employed...unless the set up is expertly blueprinted to match the cam and ignition. Today, 5 liter 19701 to 1987 Holdens with blueprinted 4MV Qjetscan make a streetable 470 hp, and 6.6 liter Ponchos with tripowers go above the 600 hp mark with ease.Each is an example of the best mec secondary carb systems ever)