Re: 305 carberated Head Gasket
First, fill it full of coolant! AS full as you can get her!!
Start it up and let it warm up AND watch for bubbles at the overflow...
Tell-tale signs of either a blown headgasket OR a cracked head AND MY MONEY is on a cracked head!! (B/c of light-weight castings head is all)...
You can crutch something like this by draining her, flushing it completely clean of coolant, then fill her up with CLEAN water and a quart of "Water-Glass", (Sodium Silicate)!
Put a piece of cardboard completely over your rad. and take her out on a 5 mile drive and CAREFULLY watch her get real HOT!!
Come back and let her cool over night and then remove the cardboard and start her up and warm up watching for bubbles again!
If there aren't any bubbles, remove the cardboard and completely drain and flush it out again and fill with needed NEW coolant!!
AND you should be good to go until it decides to let go again.
I ran a two motors like this, one for for several years!
BUT you have to flush her CLEAN b/c the Water-Glass will gum up antifreeze and clog everything up if you don't!!
Hope this helps and it is a shade-tree way of keeping you on the road until you can tear her down AND fix her right!!
PS., Water-Glass is usually what Professional stop leaks tend to have in them to use as a h-d block/head crack stopper.
And other cheaper rad. stop leaks usually just have powder filler hole plugger crap in them..