how do you know thats its 8.4:1... thats the book spec but slants can be front .125-.215 below deck. if your run the best case numbers with a 56cc chamber you get 8.89:1 and worst of 7.64:1! all over!
what you need to do is have a clean up cut done to the head so its nice and true, then cc the chambers, then measure the piston to deck height.
lastly if its a stock piston (3.4") then you can run stock head gasket of 3.4 bore by .021, there is also a steel stock style gasket at 3.510 bore with again .021 thick. now your felpro gasket will be 3.510 bore and .037 thick which kills compression BUT is easy to get, so compensate.
as Memike said off the block is miles better as your gonna get closer to quench (but never get it without pistons sticking out of the deck) but something is better than nothing!