You may have caused some serious damage to the teeth on the 1st/reverse slider gear and the counter gear by trying to get it into first while moving. I would certainly drain the oil to get the metal particles out before they finish off what was probably a pretty good transmission.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine