SAGINAW 4 SPEED TRANS ID HELP

[74NovaSS]

I picked up a Saginaw 4spd cheap I want to use in my Nova. (converting from automatic). I'm just curious as to what year it is and what it came out of. I've decoded what I could find using internet searches. The code stamp is R8P04 which is Saginaw 4spd, 1968 or 1978, Sept 04..... How do I determine if it's a '68 or a '78??? The casing # is 3925656 and it's a 10 spline input shaft with 1 ring groove meaning it has 2:56 / 1:81 / 1:44 / 1:00 ratios. Can anyone help me with the other #s stamped on the casing, and can the vehicle this came from be determined by the partial VIN stamping? Here are the other #s.

On the main case: large GM1
under that is the 3925656
under that is H278
partial VIN is 19V519922
above the shift rods on cover is GM3920798 7
on the tail housing same side is H288
bottom of main case has 35 5032 and a large B
and bottom of tail has 3873886

Another question, I've had my share of these Saginaws in the past, but how would it hold up behind a 396-454 say in the 390-450 hp / 500 ft lb range? It won't see any track time. Just a decent driver.
Any help appreciated. Thanks guys.

Ed