Jim,
LT599 is the engine type and date code for your engine. LT was the engine code for the 1965, 4-barrel 401 with 10.25:1 advertised compression in the big body cars. A few LTs have been documented in the 65 Gran Sport, but it is rare. The 555 is a date code for the engine, but I have never seen an exact decoding of it. For example, my car is LR599, and it was built in August toward the end of the production year.
5H948594 is a partial vin from the original vehicle: 5 is 1965, H is flint MI assembly plant and 948594 is the production number of the vehicle.
1964 was the last year the entire vin was stamped on the engine and it decoded differently. In 64, the first number or letter was the model, the second letter indicated the year (K being 1964), the third number was the Assembly Plant and the last six numbers were the production number.
