run the rect ports if they are in good shape but big O ports are very good 781 the gm stock head had some very good numers on them and can make big power .all the engine i have built used both O and rect . i am building a big block rigth now and it will have O ports on it this a 513 and have it is going in a heavy car and for rod length the depends what you want to spend i have built them with stock 6.135 and 250 long6.385 in short deck and in the tall deck 6.635 to 6.800 if you do not have alot of money then you can get in the 496 with out alot of money a good set of stock rods with good rod bolts in them. but the scat I beams do look good for the money$275 so at this piont go 6.385 scat rods pistons hyper pistons $285 or $550 for srps it has a lot to do with how much out have to spend but 496 can be built for not a lot of money and the o ports can and will support a 496 with some big 2.190 intake and 1.880 ex and a bowl job