we had a 406 that ran a best of 9.76 and we built a 434 that was the same parts and it ran a best of 9.68... I think it ran 6.14 best in the eighth at 3200 pounds with a 4.86 gear 32" tire.
both motors were almost the exact same heads, cam, compression, intake, carb. We ran a 1050 dominator on gas.
The 434 we ended up half filling the block and it was a little harder to cool down between rounds than the 406.
If it were me I would build the 421. you have little to no clearancing to do and the best bang for your buck in a stock block.
I hope this helps.