Sizing replacement Heat & AC systems

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I have some quotes on new system and am a little confused.                                

I have two people recommending the Bryant 80,000 BTU 95% 355CAV and one calling to the 60,000 btu.

Calculations from the contractors are 62,000 and 63,000 btus

A couple rooms have been cold in the past so I'll be adding an extra register and duct work to them and increasing the size of the main duct on that side of the plenum from 14 x 8 to 18 x 8 for the first 4 (of 8) 6" ducts.

This in itself may solve the cold problem in the two rooms but I'm wondering with the extra lines (now 14  6" lines) off the furnace if it isn't better to go with the 80,000 btu's. To complicate it further there are two 355 CAV's at 80,000 btu; one is 042080 and one is 060080. I am gathering the difference here is the blower CFM, with the 060080 being higher cfms.

On the AC there are essentially two recs one is the 186B single stage 2.5 ton and the other is the 187B two stage AC.  AC Calc was right at 28,000 btu.

This stuff gets complicated.

Can anyone offer thoughts on which way to go.

House is 1630 Sq feet in central NY and I wish to get some more heat in to the basement. I'm thinking the 355CAV042080 is the way to go. With the 3 stages and variable fan speeds I should have enough to do whats needed without being grossly over sized.

Thanks in advance for any help.