Tri-Y's will generally give you a bit better grunt down low in the RPM range; long tubes will move the range up a grand or so, and shorties won't change much over the stock manifolds. That has been my experience. I had a set of long tubes on one of my work trucks when I bought it, 400 Chevy, T400, 3/4T, 4x4, and it was a bit slow getting it going with a load in the bed and a jobsite or equipment trailer on behind until the revs got up over 2000 to 2500; changing over to Tri-Y's like I had on the other trucks made a world of difference in loaded performance - it was much easier to get a load moving, it pulled well right off idle. All three of my older trucks were essentially the same, with the 400SB and 4WD, 2 were 3/4T an one was a one ton, all got Erson "RV" cams at the first opportunity, and dual plane tall carb base manifolds and Q-Jets. They worked very well both on highway and off road, in some pretty rough service.