The tensioner blade goes in through the head. It's fiddly and frustrating, more so if the engine is in frame meaning looking down the tunnel is far from easy. Best done with the chain off.
First the tensioner should be released. Pretty easy with barrels off, harder with them on. Loosen the adjuster lock bolt, push down on the lower tensioner socket - it will move freely (against the springs) unless the horseshoe is buggered, tighten the lock bolt with it pushed down. Be gentle with that bolt though! It is not the 8mm it seems, only a 6mm where it screws in.
Make sure the rubber(?) socket things are in correctly. Thin side goes away from the chain. This is going to be tough for the bottom one if it came loose, you can barely see it but it can be set in place by sticking it to the blade with grease and fishing it down the tunnel. The upper retainer will go on fairly easily with adjuster loosened but obviously the blade will have to flex a fair bit it that isn't done. Since your barrels are raised anyway maybe you can see the lower blade socket to check the black thing is in OK?
A new base gasket is advisable but you could clean it as well as possible and maybe add a bit of Hylomar or whatever (NOT silicon or that Permatex black stuff), hoping for the best.
Reinserting the pistons is actually rather easy. Put a pair at TDC, align the ring gaps to not align with each other, not in line with the pin, not perpendicular to the crank. Gently slip the barrels down while working the top rings in with your fingers: there's a nice internal taper in the bore that will get the rings in if you can push them in enough to enter the tapered end. The barrel section will rock enough to work on one piston then the other one ring at a time. Once all the rings on that pair are in, carefully turn the crank while walking the barrels down until the other pair comes up to their bores. Repeat the insertion process... not as much room for the fingers but you've just had some practice so it should go OK. Doing 2-3 first gives more finger access at 1-4 for the second set.
If this is not working for you, properly sized gear clamps make reasonable McGuyver ring compressors. Don't tighten them beyond where the rings enter those tapers, and coat them with oil. Same process of doing one pair then the other, trying all four at once will just drive you mad.
Be aware that current asbestos free head gaskets are thicker than the OEM factory installed one. This makes the barely functional "orifice control valve" seals even less capable of stopping the head gasket outer end oil leaks - almost guaranteed to leak if stock seals are used with a new head gasket.
Download the service manual. The online scans are not excellent but better than nothing. The Haynes type books have some better pictures but you should have the real manual as well.
