There shouldn't be any mods necessary... The 289/302 are only 70 pounds heavier than an inline 6...
Kind of like keeping the straight 6 and having a fat chick in the front seat... You'll hardly notice.
Unless it's an oil pan clearance issue which is unlikely...
Oh they are very different. But only in the motor mounts themselves. All the others holes, a-arms, R/P mounts are the same. You have to swap out a 6 cyl one for any other. 2 members where used, straight 6 cyl and all others. Easy bolt in setup. Any Fox Stnag stuff fits also. I have a tubar D&D K member and A-arms I need to sell. Problem is I can't ship it for a few months. email me if interested. I also have a 175# coliover setup and BBK caster plates, brand new, never used. And a limited time offer, spindles and 5 lug rotors, too. Blue light special on Isle 5.
I've heard of people modifying the 6 cyl. to fit a V8 but imo its not worth it, there are tons of these cars at salvage yards (the secret to building one is now out) you could just snag a 4cyl. or V8 subframe from a Fairmont or a Fox body Mustang.
I went with the D&D tubular Mustang setup to mine.
that's what I did. It was VERY simple. I had to cut off the front part of the motor mount "ears" as they are much longer front to rear than the V8 or I-4 K members are....on one side, the oil filter hit, and I believe on the other side the fuel pump was hitting (belt pump) so we just trimmed them off. At first we used stock 5.0 engine mounts but now we have a pair of custom solid mounts that we just made here at home. The 5.0 mounts aren't quite a PERFECT bolt in, but very close. Ours was originally a straight 6 ('78 Futura 250/C4). Even the headers clear...but just barely in some places, and they are 1 3/4" hookers, which BTW, actually fit better than the Hedman 1 5/8". Plus, they're about .1 faster in the 1/8 mile. Combine the headers with alky injection and a Victor Jr intake, we picked up close to a half second in ET and nearly 7 mph. Went from 7.20's to mid 6.60's.
The I-6 K member WILL work, but it ain't a direct bolt-in affair. The 4 cylinder and V8 Kmembers are a better fit and a tube K-member is even better...which will be my next project on my Mustang when I win the lottery.