Big block cooling issues

yep, 10" fans are too small. You need at least 12" and try to fit 14".
Here's the results of some tests we did when designing our fan kits.
dual 12" 2800 cfm
single 16" 2600 cfm
dual 10" 1400 cfm.
Doesn't matter what they are rated at, there's not many 10" fans that pull much air unless they have huge motors on them.

Upgrade the fan/shroud combo and that should bring the temps down quite a bit.