I did the same swap in my '69 Firebird and used a fresh three core rad. No cooling issues (probably because of the cooler Alberta weather!) with any of my engine combos up to about 390 hp.
When I had a 350 swapped for a 400 the one tricky part I remember is that the Ram Air repro manifolds had more mounting holes than the heads had holes for. Two (or Four?) holes per head had to be drilled and tapped. Your situation may differ...
And if the head holes are "missing" for the exhaust manifolds (should not be on '71 400 heads) it only makes a difference if you are using headers. Stock manifolds are just fine as is. I haven't tried it with long branch manifolds however.
Vikki 1969 Goldenrod Yellow / black 400 convertible numbers matching
Got the 350 pulled today and since the complete engine appears to be original, I'm keeping some of these parts with the original block and some I just need for the 400 we're building. - timing cover (11 bolt) - 6.75" harmonic balancer - timing indicator (2 bolts, mounts on pass side) - fuel pump eccentric - lifter galley pan - complete dipstick (innner & outer tube) Thanks!
Spiderman, I should have everything except the 11 bolt cover and the tubes.I will have a look.
David
http://FirstGenFirebird.org/show/closeup.mv?CarID=571 If i don't get this car back on the road soon i'm gonna go postal! On a quest for FGF knowledge 1968 Pontiac Firebird Convertible 1969 Oldsmobile Cutlass "S" Convertible *Sold*