The 402 can be a formidable player. I have one the came factory in my 72 monte carlo. I rebuilt it a year ago and it runs well. I wanted to maintain the factory appearance with the cast intake and snorkle air cleaner and cast manifolds. I went .030 over, KB hypereutectic pistons w/ .180 domes ( stock domes) arp rod bolts, Heads are casting 292, they are the same or similar to the 840's you have. I pocket ported them and cleaned up the runners and bowls. I installed stainless valves 2.06 int and went to the larger 1.88 on the exhaust. This helps scavenging, so i am told.113cc chambers yields 8.8 to 1 compression. Cam is awesome. Crane 272 powermax. 216/228 .515/.510 112 lsa. Small cam but big car.. Has mild lobe sound. Cast iron intake w/ 750 rochester done by jet performance. When I raced it last year, I ran it twice and both times spun the tires through 1st and part of second. 1st run- 14.81 second run 14.38. I did run the car with headers but quite honestly, with the manifolds on now, I don't notice any difference. I'm confident that if I could hook, I would easily be in the 13's. Not bad for a 8.8 to 1 compression mild street engine in a 3900 lb car.

I f you want to use that short block, at least change the oil pump and measure the pickup distance. I would also go with new rod bolts but that involves dissassembly. Take the 840's and cc them. Hopefully they are 113. if they are and you have the stock cast pistons, you're looking at around 8.5 compression. Although a bit expensive, go with a small hyd roller as this will really wake this motor up. Howards has a few good candidates and they are sold at competition products. The 245 and 255 part numbers are great choices. You could probably get by with the 255- it is 230 at .050 and I don't know the lift off hand.
You should really talk to the grand pu bah of chevy build ups. He is Pat Mccarthy and he is on this forum frequently. He says 402's are great eng if you run a small roller. It really wakes them up. Also he says that larger intake valves may not make a difference.