which engine

i am going to be aquiring a 83 - 86 chevy half ton 4x4 pickup. it has a 6 inch lift and has 35/12.50/15 tires.
i was wondering which motor i should go with it has a 1970 350 with alum
intake and headers in it and i have a 400 engine ready to rebuild in the grage that one is a 76 i was wondering which would give me more power. i know there was a power drop off in the 70's somewhere either one i use has to be rebuilt.
and any other relatively cheap mods i could do to either to invcrease power.
thanks for your advice.
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