Chainsaw Repair

I am NOT a Poulan guy but own two, 3000 and another that I cannot remember what it is. Anyway, I got a 3400 top end from another member here for $15. I think I remember reading that the 3000 & 3400 are the same saw. So, can I swap the 3400 top end on to the 3000? And is there anything to gain by doing so I mean besides .4 CI?

I'll bet I know where you got that idea Tim.  There were some grey/black Craftsman saws of the 3400-4000 series that were labeled "3.0".  These were different than the "3.0" labeled Craftsman saws of the 3000 family.  The two saw families are easy to tell apart.  One major difference is that the 3400-4000 series all have the mag rear handle, while the 2800-3600 (etc) family that the 3000 belongs in has the plastic rear handle/fuel tank casting.

The 3.0 labeled 3400-4000 series saws in fact WERE 3400's (you probably read that before and mixed "3.0" with "3000").  Same displacement, but 'falsely' underrated for a price point.  A case of getting more than you paid for.  Those have a different Sears 358.xxxxx model number than the 3.4 labeled 3400's, but they share piston/rings/cylinder part numbers with the 3.4 branded 3400's  The 3.0 labeled 3400's seem to have 'numberless' starter cover and clutch cover decals (with the bar size and displacement labeling instead being on the bar............such as "3.0-16").  The 3.4/3.7 labeled saws in this series usually have the "3.4-16" or 3.7-18" integrated into those little striped cover decals....