You will destroy the drivetrain if you swap to the 350, the stock toy axles although tough will eventually explode as will the driveshafts.

Now, if you swap to D44 or D60 front and a corp rear or ford 9 inch or similar and do it that way it will hold.

If you want a super capable and ultra reliable truck I would find a 87 and lower chevy/GMC and pull the drivetrain and put it on the toy.

Otherwise stick to swapping in a V6. The old buick 3800 is an ok choice but they vibrate a bit, could go with a 4.3L v6 from a chevy/gmc or swap to a toy 3.4L 6 or inline 6.

Could also keep the 4cyl and do a doubler and have the crawler transfer case setup. It aint gonna be fast but you will be able to crawl and put down massive amounts of torque.

I wheeled with a guy who had an 85 SFA 22re done up like that with 36 iroks and it was UNSTOPPABLE!.

Bottom line: It aint gonna hold up with the stock toy stuff under there if you swap for the v8.