77 is a bad year core.
Find a 1973, 1974.....or skip the crap and go to 87,88,89,90,91.
The crap being....high mileage, smogged down, never tuned up RV/Truck engines.
Find a wrecked Ramcharger 4x4.
Find a Wrecked D250.
Find a Low Mileage 73/74/75 Rv.
Whatever you do....dont spend more than $150.00 for a core 360