>>76349155 (OP)Manual labor is actually terrible for gaining muscle. Unlike the gym where the equipment and exercises are designed to keep you safe and target the muscle directly, everything you do in manual labor will be awkward, strenuous, and painful.
Most manual labor is like aggressive cardio with occasional attempts to fuck up your joints and destroy your back. The average gym goer can grow more muscle in 3 hours of lifting per week than 40 hours of manual labor a week.
Most blue collar guys I know who are jacked STILL hit the gym. If you don't you'll just be skinny fat with good grip strength, good body positioning, and a bad back. You will have some muscle, but it'll be very hard earned muscle.