The car may not have the original engine in it. There are 3 sizes of "A series" engine 948, 1098 and 1275. The 67 - 74 is supposed to have a 1275, but they will swap and bolt up no problem. If it has a pipe coming out of a cover on the side of the block under the manifolds, then that's the crankcase breather and you have a 948 or a 1098 engine installed. See the diagrams on "engine external" in the Moss online catalog to ID your motor. I don't think its possible to swap heads from a 1274 to a 1098, but someone may have tried and actually got it to run
If it has a breather on the side facing down, then the most likely culprit is too much oil, or oil with a lot of gas in it causing the crankcase to be overfull.
They only hold about 4 quarts. If the oil smells of gas, change it! Others here have warned of the need for a zddp additive to the oil due to modern oil chemistry being changed. Do a search if you don't know about it already.
As to dropping the tranny, forget it, the tail end sits on top of the frame and you'll never get it out. You can however, remove the engine and leave the tranny in place, but you may not need to. If its overfull with oil and you plug the breather, it'll blow out the back. There is no rear seal. Seal softeners and oil additives will not stop the rear from leaking because there's nothing there to soften.
Hope this helps.
-Michael