Pop the muffler. If he's uncomfortable letting you do that, pull it over to get a feel for compression. If the piston gives you a lot of good feedback (strong pulses as you pull the rope = good compression), the top end is PROBABLY ok. Nothing like a visual inspection, though. You'll immediately see flaws in the piston if they are there. Do all this, BTW, using words like "hmmm," and "huh." Sigh a lot and make little clicking noises with your tongue or something. Nothing more.
If the top end is good and he goes for the $50, you stole it. If the top end is bad, $50 is still a good price, just not "laughing-madly-and kissing-the-ground" good. I would then go about looking for a used top end. You might need to have patience until the part shows up.
One of the other guys would know whether there are less expensive aftermarket top end kits for the 350. If there are, a new aftermarket setup will probably cost you $100. So your 350 would cost you $150, which is a decent price on a good used runner.
Keep us posted!!