If the whole point is that you want to run headermufflers right of your 3½" collectors just get a 3½"-3" (or whatever size glasspack you want to run) reducer with the flanges and clamp or weld a glasspack to it and you have made yourself a headermuffler that fits your desired collectorsize.
Its not realy rocketsience,but if you use a reducer with rotating flanges dont forget to locate the flange on the right side of the flare before you weld.
Here is an example of products if you want to run 3" glasspacks/bullets/headermufflers.
If it where me i would get reducers with welded flanges. I prefer Hookers reducers. as for the mufflers/glasspacks/bullets,Dynomax racebullets have been used for along time behind hot engines and dont have louvers punched the wrongway into the flow but other options might be just as good,just avoid the ones with louvers that screw up the flow.
https://www.summitracing.com/int/search/department/exhaust/section/header-reducers-extensions/part-type/header-thanks....reducers/collector-diameter/3-500-in/reducer-outlet-diameter/3-000-in/quantity/sold-as-a-pair/collector-attachment/3-bolt-flange?N=4294948820+4294900450+4294896645+4294897470+4294866854+4294951454+4294924062&SortBy=Default&SortOrder=Ascending&retaillocation=int
https://www.summitracing.com/int/search/department/exhaust/part-type/mufflers/section/mufflers-resonators/case-shape/round/inlet-diameter-in/3-000-in/outlet-diameter-in/3-000-in/inlet-location/center/universal/yes?N=4294948820+4294948821+4294948819+4294910233+4294900371+4294900370+4294910231+4294951518&SortBy=Default&SortOrder=Ascending&retaillocation=int