My suburbans motor has a knock,(350 TBI) it misses at 15-35 mph but after that its ok.. I just had the heads re-done. My Grandpa said because the lower half has high miles about 170k The top half is seating really well and giving good compression causing the lower half to go due to the mileage and wear and tear from trailer towing. So I guess its safe to say it needs a re-build. Is It easier to just buy a new one? or have it re-built?