The 200-4R is a good combination with 3.08 and numerically higher rear end ratios, if performance is your goal. Keep in mind that the 1st gear ratio is 2.74 (about the same 2.48 as the TH350/250 that was used for decades with rear gears down to 2.14).
If you are not already down the road on the transmission, pause and consider the 700-R4. It was designed to get big barges off the line and still give good gas mileage by using a 3.06 first gear, then similar ratios on up. A 700-R4 works well with ratios well into the 2s, so no additional expense to change the rear end - you can drop in a 'lunchbox locker' yourself without pulling the differential out, assuming everything else is behaving.
The lockup kits for the 200-4R and 700-R4 are similar in price and complexity. Jegs and Summit sell drop-in 700-R4 crossmembers, while the 200-4R versions are more complicated with fewer options. The driveshaft on a 700-R4 will definitely need shortening 3". I have one in my garage from the last time I swapped one. But if yours is old and rusty, you'll want to go with new U joints anyway, so give it some thought.
Both transmissions are about the same size, although the mounts are in different places. The 700R4 is much more common, and probably cheaper to build. Its downside in performance applications is that the jump from 1st to second is pretty large. Plus, if you go with lower gears later, you'll blip through first gear pretty fast, negating the big advantage. A rear axle rebuild generally ends up in the $900-$1200 range once you pay for the labor to do the swap. 7.5s are not that strong, so if you go this way, plan on spending $1500-$2000 towards a Ford 9" conversion or hunting for a rare 8.5 GN/HO axle (still $1500 to find and then upgrade one).
Conclusion: think about what else you plan to do down the road. What is 'enough'? If you want to make your stock ride snappier and will not go race with 600 HP, then you might want to keep what you have and go with the 700-R4.
