481988 is the block casting code. It was used on 400 engines from 1971 through 1975. All 400s, not just YT code.
YT was used 1966-1969, 1972-1976, and 1978. Just those two codes on the same block narrow it down to 1972 through 1975.
The 5c is the casting code on the heads and was only used in 1975.
The intake manifold part number and casting date are 1975 model year.
My guess would be that you have a 1975 engine. I am not sure what models used the YT in 1975, but the partial VIN can tell you what assembly plant it was installed in, which narrows it down. It was a 185HP engine, not exactly a screamer.
Vikki 1969 Goldenrod Yellow / black 400 convertible numbers matching
