1 3/4 primaries. i got the kit from speedway motors they have both 1 5/8 and 3/4. its an 86 jimmy. not sure on the t56 but i hope they clear cause i would love to have one when my 700r4 gives up. the set up is a 5.3 i ported the heads, converted to carb. and it has the gm ASA cam ( little bigger than the hot cam) i havent fired it up yet but im lookin for 450-460 hp i also match ported the intake and im running the msd 6ls box and a ls2 pan with plenty of clearance.