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Can somebody tell me what kind of gains would i have
Boring a 3100 engine 30 over with a 3400 top end swap including 3400 heads as well
Is it worth it?

The boring would be negligible, however the 3400 (or 3100 VIN J) top end has significantly larger intake ports. The Difference between the M and J 3100s is about 15 HP and FT-Lbs, not to mention increasing the power band.

Thanks for the info

My '95 Z-26 had a 3400 intake and TB on it, it definitely had more punch to it. I was missing the roller-rockers (and had to modify the lower intake manifold to clear the taller pivot-ball rockers from the 'M'-Code engine) used in the 3400s and 'J'-Code 3100s, which are probably good for 1-2 HP just by themselves.

The increased throttle-response was across the entire power-band, and I didn't encounter any negatives at all with the work on it. Worth it? I don't know if I can answer that, but there is a noticeable difference in performance for sure.
Edit: X2 on what Cliff said so far as the .030 over-bore, I wouldn't bother with that unless the engine legitimately needed that much machining to be right again...