73 340 heads are J heads....just replace 1.88 with 2.20 intake...then you have a 71 J head.
The ports are the same...
exhaust manifolds are different...put headers on it if you want the best performance...
you need to raise the compression....KB243 pistons will get your compression back to 71 level....although the 71 340 is rated a 10.5 to 1 compression is it more like 9.5.....and with cast iron heads is probably the best you should have given today's gas....