You canNOT concatenate different lrecl FB files. You need to process each one at a time. You can use a single ICETOOL step with DISP=MOD on the output dataset and copy all the files with a COPY operator.
Yes assuming 300 is the longest length 80 is the least you need to concatenate file with 300 lrecl followed by 80
Guess There are some post's discussed already
Did you read Op's Post clearly ? He mentioned his input files all RECFM=FB files. AFAIK none of the sort products support the concatenation of different LRECL FB files.