for a 1977 from frame to frame the width is 24.75"  with center point being 12.375" and your mounts will join centered with protrudance of 2.75" left and right......... .. 

Six times for the first time might be a low count , My first one back in 72 was more than three times that and scraped all the paint off the fire wall trying to get the oil pan to clear the rack and make up mounts , cut up quite a few oil pans until i figured it out , then I see a few other helpful tools today that I wish I had back then , like this fake plastic engine block ,yep they make fake heads and belhousings too, live and learn from the mistakes of others ,

your probably right when you have the whole setup it would cost as much as a engine ,, it would only have the advanage if your going to do a dozen or more engine swaps using the 302 , nice to have though , but if I would just win the Lottery I could buy a the 351 version to get ready for and bored and stroked to 427 all aluminum engine with a six speed , but just saying if I do maybe , hope , wish , etc