Advice or Input needed. 3.42s w/ "32" tires.

[NastyBuzzard]

I have a 2004 GMC crew cab/short bed 2wd. I just got this truck. It has 275/55/20s and has had the speedo corrected for this tire size. The tires are about 32" tall and I have a 3.42 rear gear.

I currently have the 3.42s and 32 MTs on my 87 4wd and I know from driving it that the rpms I am running are too low. The 87 has to try way to hard after OD kicks in.

I have noticed that the rpms while I am doing 60MPH in the 04 are only about 1500 and sometimes when I slow down to 45-50 they stay around the 1100 RPM range.

I have a feeling this is too low. I know lower rpm=better MPG but at some point the lower rpm will make the engine struggle and not be in its power range and then its wasting gas.

I have a 2001 z71 donor that I am using for a ls swap in my 87. It has a 3.73 posi (g80) rearend.

What do yall suggest? Run with the 3.42 open rear or swap in the 3.73?

The 04 will only see light towing, it is mostly a 4 door vehicle that all the family can fit in.

Here is a pic. Anyone ever seen these rims before? I haven't, I checked and they are a GM rim. Maybe off a SS or Denali?