I just put a used GM Goodwrench 350 in a '77 Crew Cab 4x4. The truck is HEAVY, but I'd have figured the 4.56 gears in the rear and the only 29" tall tires would still allow it to tow ok. On short hills, I'd go from near 60mph and 3500 RPM to just under 40 mph before I'd even reach halfway up the hills (like 1/2 mile long hills if that). And that's floored. The motor runs excellent, but it just doesn't seem to like pulling a huge load around. Coming back home w/out the trailer, it was able to maintain speeds up the hills floored. I guess I've taken my big blocks for granted. My "slug" of an '81 Crew Cab doesn't seem so sluggish now.