This topic would be better suited for the hipo shop.

If you have a 41 transmission, it will not directly bolt up to a sbc 350. You will have a fair amount of work figuring out the brake and clutch pedal setup. The brake and clutch pedals mount to your bellhousing now.

There is nothing wrong with keeping a 6 under the hood either. If it were mine, I would build a 261 and leave everything behind the engine alone. Of course if your truck is the gmc version, which I think it is, it would likely have a 270 motor. I would strongly consider rebuilding that motor or replacing it with a gmc 302.