Weird Transmission problem?

Hey everyone I bought a 87 ram 50 with the 360v8 and 727 automatic transmission swap about 2 days ago.The issue I’m having here is the truck shifts through all 3 gears just fine but it tops out at 40 miles per hour with super high rpm’s,Where should i start looking to fix this problem?
(1st -2nd gear 0-20mph 2nd-3rd gear 20-40mph)
Author: Ram502023