not without a scanner that can read the Knock Retard.

3.4 is in the grey area, some can and some can't which is why you need to keep an eye on KR when you try it.

Have you any other mods? Ported Exhaust manifolds, Rockers...?

What timing do you run right now at WOT at the top of 1st and second? What KR do you run at the top of 2nd and bottom of 3rd?

If your timing stays around 15* and your KR is around 0, You are safe to drop down. If not, fix the other issues before dropping down

I could run a 3.4 with no problem on my 98, 3.3 with minimal KR. However on my 2000, It does not like the 3.4 at all.

If you have 0 KR with the 3.5" I would say it is safe to drop to a 3.4" . Keep scanning though, you didn't mention which plugs you are using but I'lm guessing they are 1 heat range colder.

Ed

It goes without saying that you'll run PREMIUM fuel only (91 octane minimum, 92 or 93 preferred), you'll keep watching that KR number, ESPECIALLY on hot/humid days.

Scanning for KR on a cool day or even an 80 degree day is pointless. Hot days can kill you.

605s are one heat range colder I believe, 104s are 2 heat ranges colder. I run 104s with a 3.4 with no problems.