781's are Large Oval Ports not Rectangular with approx a 118cc Combustion Chamber (Always CC BBC Heads to ensure the Volume as they have been known to differ). The Guides are Critical and you can install larger 2.19/1.88 Valves with moderate Bowl porting. NOTE that the Larger Valve tend to decrease the Chamber Volumes by a cc or 2 but by the time you un-shroud around the Intake and Venturi approach the Exhaust your back at 118cc's.
With Flat top Pistons the static compression is approx 8:1 - very low for a Performance cam so do not use more then a 266 or 270 cam. Alternately you can use a 396 Close Chamber head with something like a 98cc Chamber Volume over them Flat top pistons to raise the Static compression to approx 9.2:1 and use that 274 cam with close to an 8:1 DCR.
You're best is to replace the pistons with something like Speed Pro L2465F30's with a 25.7cc dome with 781's which will bring the static up to approx 10.2:1 using the ,018" thick steel shim Head Gasket - in which case you can run a verily stout cam like a 278 or 282 providing an 8.2:1 DCR and run Pump Gas - Here is where the 781's really shine if you shell out for a SR Cam ; however, I recommend the XM278H-12 and CC-925 springs with nothing but a Piston Change & 0-Deck for your 1st BBC build as it prepares the Block for Future changes if you so desire. 500+Hp & 6400rpm isn't shabby for relatively Stock 10.2:1 HFT BBC - Roller Rockers (Harland Sharp or CC-Pro-Mags and 2" ARP Rocker Studs are a must spinning over 6000rpm ;o)
You can 0-Deck the block and run a composite .038" Head Gasket - which is handy if you ever go to Alu-Heads. It is important to keep the quench between .034" to .044" with anything over a 9:1 Static Compression - particularly with Iron Heads.
A Cast Crank is just fine to 6400rpm but if you thinking 7000 rpm & 600+Hp you need a Forged one. With prepared 781's, Forged Crank, Auto-Hole Shot to 3.73 Gears and 10.2:1 Static you can get away with a 282 to a 288 SR Cam and put out 600+Hp with 7000rpm revs - of course it's expensive.
You should check that 959 Block as it could possibly be a 4 Bolt with a Steel Crank. NOTE the Engine CODE and find out what type of Vehicle it originally came out of.
PS: Don't post so many threads, members can't keep up to what you have and don't have ;o)
P. Kelley's DCR Calculator and Comp Cams Camquest Software are handy apps to help you design the block and choose a cam: