I have heard of Thompson, never dealt with them directly, but reviews have been positive.
On that motor, I am not entirely sure I would want to take a 5.3 and bore it out to 5.7. I know the iron 5.3s have thick cylinder walls and can take it, but I have always been against enlarging cylinder walls, just to enlarge them.
For the price you are going to pay for that one, why not look for a completely stock 6.0 with low mileage. If you search hard enough you will be able to find one with under 10K miles, that technically hasn't even been broken in yet, for less than the 5.7 you are looking at.
Your car, your money, just my opinion.
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'89 IROC-Z Turbo 3.8 5SPD
LS1 Brakes|Wilwood Prop Valve|LS1 Master Cylinder|Aluminum Driveshaft|LS1 3.42 Rearend|Adj PHB|Adj LCA|LCARB|Koni Struts|Koni Shocks|Adj Caster/Camber Plates|STB|10 Spoke SS Wheels|PHB Relo Kit|Adj TA|SFC|ARAP Based Prom|B&M Short Shifter|Underdrive Pullies|SMOG!|AIR!|Rag Joint!|Magnaflow|Ported Plenum|Misc Stuff
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