will a1995 4.3 interchange with a 1993 4.3

i was told a 1995 4.3 will not work in a 1993 truck why will it not work

Will it "work" ? Most likely Yes.
Is a 95 exactly the same as a 93 Probably Not.
What is wrong with the 1993 ??
If you want any usefull assistance here, you will have to provide a lot more information about the two engines and what they are in.
Look at the '95 timing cover -is it tin with 10 bolts, or plastic with 6 bolts.
Are the fuel systems TBI or "spider" injector.
Can you tell how many knock sensors are on each engine, and if they are in same locations?
Read this.
http://www.enginebuildermag.com/Article/2427/rebuilding_the_new_chevy_262.aspx