You'd need to gut the Olds tranny and install the 4x4 output shaft in order to use the Olds engine,or use an adapter plate between your existing chevy tranny and the Olds engine (about 70 bucks from Jegs or Summit)...unless you happen to have a rare dual bolt pattern bellhousing tranny already..
I have a friend who has a '69 C-10,when he bought it someone else had put a 350 Olds rocket and TH400 trans in it from a crashed '72 Cutlass,it ran very well ,but he is a bit of a purist,he put a 350 he had back in it with a TH350 that was built up...the truck also had the Olds Cutlass rear axle swapped into it,he ended up finding a chevy stock 12 bolt posi for it instead...(truck was like showroom condition,he said he would not have bought it otherwise due to the drivetrain mods,but whoever did the swap did a decent job)..
He sold the Olds engine on craigslist ,only took 2 days before someone offered him 250 for the engine,and he gave the guy a deal on the tranny,for another 100 bucks...
Olds and Pontiac engines are getting scarce around here and sell quickly,even if in need of rebuilding,you cant even find a good core usually..so if you decided not to use it,you'd probably get your 300 back fairly easy if they aren't common in your area still...scrap is down though,a complete V8 will only bring maybe 50-75 bucks for scrap around here...I'd say it'd pay off to put ads in craigslist and hemmings and you'd have no trouble selling it...especially a 455,muscle car guys are always looking for them,running or not..
just sayin... 
