I am considering change out my m21 for a 5spd.T5??? What swaps out well? Which transmission should I be looking for? Is the Mustang T5 world class a tougher unit? And will it mount to the GM bell housing? Are there any imput shaft concerns?? Will the shifter be in the same location??? 67 RS 350

no, dont get a weak T5
you need to get a TKO 600
go to
http://www.classicchevy5speed.com/67_69%20Camaro_Firebird_Tremec_Kit_Selection.htm

use the search function of this site, you will find tons for information to back this up
good luck