I think he means that the turbo used on the auto merkurs was smaller than the turbo used on the manual merkurs. He is right, the manual merkurs came with a 0.61 A/R hot side T3 while the autos got a 0.48 A/R hot side T3. Both of these turbos are larger than the IHI turbo that came on the 87-88 turbocoupe.
The 0.48 T3 is plenty of turbo up to about 300 hp. I am running the 0.48 T3 on my engine (about 270 hp) with the ranger M5OD. It'll spool at about 2500 rpm (vs. about 3300 for rpm the 0.61 A/R model) so it's nicer to have in a truck. Even with the smaller A/R, it still breathes fine at 16 PSI and 5500 rpm.
That setup will work great for you. Do use the ranger's tranny though, it's much stronger than an A4LD. Also you can use the auto computer with a manual tranny. The only difference is that your idle will be about 1100 with the clutch out and 700 with the clutch in. (The clutch switch on a manual is connected to the same pin on the computer as the transmission range sensor on an auto, so the auto computer thinks you're shifting into drive when you let the clutch out, and speeds up the idle to account for the additional load that the torque converter would put on the engine as an auto tranny was put into drive. Since there's a manual transmission and no load is put on the engine when you let the clutch out in neutral, the idle speeds up about 400 rpms)