Not exactly a Topkick but heres some shots of the GM 350 engine in one of our 22' Isuzu box trucks at my office: She aint pretty but today was a birthday of sorts for her, she rolled over 350,000 miles without a major mechanical issue. At this point she's had 2 alternators, 2 fuel pumps, left side exhaust manifold, 4 injectors, plug wires X2, plugs x4, and the A/C compressor replaced. She's been treated to oil changes every 7000 miles needed or not, and coolant changes every 40,000 miles. She currently suffers from ticking lifters, slipping trans, looses 1qt of oil every 1000 miles, and the HVAC fan suddenly got finnicky about when it turns on. Thats the factory 700R4 behind her which has never failed, it gets a trans fluid change every 40,000 miles. The truck is a 2002 Isuzu NPR that was recieved November 2001 with 47 miles from Fort Worth Isuzu Industrial Power along with her sister that currently operates out of the shop with 297,000 miles and a similiar maintenance and repair record. Both trucks are driven daily anywhere from 10 to 200 miles carrying an avg of 5600lbs in city and hwy traffic, both avg 12mpg. She'll stay on the road until the engine dies, maybe longer depending on repair cost. So the answer to your question of how many miles can I expect from my GM engine is I dont know but if you take a moderate amount of care it'll last a long time.