I'd go with the 421" like other said very little clearancing on the blk, also if you run Eagle rods, they use 7/16" rod bolt heads which helps even more on the clearancing. We have a couple 434's with stock blks and so far no problems, in fact the stock block is quicker than the after market 434" by a tenth 4.80's VS 4.95 in 2 different dragsters, but it's filled to the water pump opening and we run a main girdle.
You will have to run a small base circle cam with either one 421" or 434" and some cam companies call .925 dia a small base circle, but in reality you'll need to be below .900 dia. My 434" Bullit cam is .875 in a dart little "M" blk.
4.86 sounds good with a turbo 3.50 trans, if it were a PG, i'd go 5.13 or 5.29 gears, for the 1/8 mile runs only.
JMO
Zip.