Thermostat.

[Arekusu]

I'm going to put a vote in for timing and air fuel ratio. Having just done this (posted in the build thread) It helped me loose about 15 degrees on my big block.

I didn't see if you just put this truck together or it has been running for a while but you may be running too lean which will cause the higher water temps. So I'd check the initial idle mix after the timing is set then check the jetting to make sure you're right. You could check the plugs or if you had an O2 sensor in the collector you could check the Air fuel that way with a wide band gauge. This is assuming your cooling system is legit , your not running crazy compression, and your cam isn't radical.

My .02

Alex