Did engine swap now RPM's are higher at cruise speed
 

Guys i need some help, i'm new this site, i have a 1998 GMC 4x4 that i had a 4.3L v6 taken out and 5.7 350 put in, new crate engine. I bought a used wireharness for the v8 and got another ecm and sent it off to have a gas saver tune put on it. Now that the new engine is installed, i am running more rpm's at cruise speeds, i'm talking 2000 rpm at 55mph, 2300 rpm at 60, and i have driven surburbans and tahoe and pickups with the 5.7 and not seen rpm's high like this at cruise speed. i used my same flex plate and torque converter from the v6, the tranny is a 4l60e automatic, can somebody shed some light on what is happening? The tranny shifts fine, does not feel weird or nothing but why am i turning more rpm's with a bigger motor to go 55 and up???

thank you