408 or gen 3 hemi

Based on a quick internet search, the SB block is ~170 lbs and the BB block is ~234. Low deck blocks seem closer to the 200 lbs mark. So the difference is roughly 60 pounds. A 5.7/6.1 block is supposed to weigh around 165-170lbs.

Final build weight will vary, but with all alloy parts and headers a BBM can be built to be pretty close to a SBM weight. It'll make plenty more power enough to justify the weight IMO - but we all know what they say about opinions.