Do you have a shroud on it? How about the filler panel from the top of the rad to the grille support? Without that piece, hot air recirculates over top of the rad and back through it. I've seen big blocks in those old trucks with stock, 283 rads with no cooling problems.
Is the truck slammed? Sometimes if they're really low the air can't get out of the boiler room.
If you need a new rad anyway, get the biggest one that'll fit. A GM diesel pickup rad will cool it, if it'll go in there.