I've never dealt with S & J, but I'm sure they get some business with a 7 year/ 100k mi. warranty. Sounds like a nice warranty, but unless you're local to them you'll have to eat the cost of shipping a defective engine back to them- not a cheap or easy thing to do. I'd rather have a crate engine from a well known Pontiac builder or the build from a local build with a good reputation and current references- much better than a piece of paper.
As far as cores go I do know that if you don't have a 455 core to exchange for a rebuilt 455 they will charge you- 455's are somewhat rarer than 400's, being built only from 1970-1976, so expect to pay more for a 455 than a 400. I think they also have a local core service to save on shipping costs.
I'm not sure about the 70 9799140 455, but I do know the 71-74 485428 455's and 75-76 500813 455's have 5 mounting holes per side, and will bolt into a 76-81 F-body chassis with no mods. Your 350P timing cover will bolt on a 455 with no mods, but I would forget about trying to re-use your harmonic balancer- a 30+ year-old balancer can fail without notice due to the brittleness of the elastomer between the hub and outer ring. Get a new GM, Pioneer, or Professional Products balancer- these are all inexpensive and a must for any performance engine. Other upgrades that are also worth the expense are forged pistons and rods, as well as having the rotating assembly balanced- a 455 has much higher piston speeds than a 400 due to the long stroke crank and the factory cast rods are subjected to much higher loads at speed. So many 455 owners claim they don't race, but one thing leads to another and the next thing you know those cast rods are getting tossed out the side of the block or through the oil pan- a set of 5140 replacement rods are also very inexpensive, around $240/set. I would also plan on beefing up your trans- the 455 will bolt up to it, but anything built for a 350P isn't strong enough to handle the torque of a hi-po 455, at least not for long.
