I have to second what Jeff has to say - you are so much further ahead with a 302/5.0 engine. You already are aware of the transmission match up problem. A 289 can be a great engine, but has some limitations that to overcome are, for no better words, a pain in the @$$

If you need heads the 302's fit fine but you lose some significant compression ratio which equates to effiency(gas mileage) and performance(ho-hum runner). The original 289 heads had +/-55cc comb. chamber where the 302 size engines had 63 to 69cc. Also, no-lead valve seats were supplied after ~1974 - 5