If you are just doing the dash speakers you should be fine with the above. Very simple swap. If you purchase from Crutchfield and enter your truck details, they will include the proper metra harnesses in your shopping cart (for free). Plug and play. Easy.
Those infinity’s come with capacitors (bass blockers) that if you wanted to use you would have to splice/solder them inline with the metra harnesses. Some people do it, some don’t. The bass blockers purpose are to block low freqs (bass) from passing to those small dash speakers and overpowering them (sounding like garbage or blowing them). But since you have the alpine, I believe the amp in that system handles the crossovers (so the amp automatically blocks low bass freqs from going to those dash speakers)......so, you shouldn’t have to worry about using the bass blockers.
Since you have the 9 speaker system you have 3 dash speakers because you have a center channel. So you will have to either order two sets of speakers and have an extra, which you can post for sale in the buy/sell section, or you can check to see if anyone here has an extra they are selling since they did the same thing you are doing.