42 2 ton is an open driveline

you will need to weld pedal mounts onto the bellhousing to use your 42 pedals

you will probably need to move the battery box because of the SM420 hump on the passenger side

get the hydrovac system off the 56 while you are at it

if its 270 gmc into a chevy, make sure you check the length and/or read Tony's 302 gmc into a chevy article

should be a simple swap