Gen 1-4 are pretty close dimension wise. I'd have to check my Gen 3. I think the Gen 1 is a little more compact due to the induction setup. Width is not an issue (similiar to a small block).

Be prepared to heavily modify your car. Firewall, trans tunnel & cross member, K-frame. Best bet is to go with an alterkation front end then make custom mounts and tranny cross member. You'll find the stock steering box wont wont work - but with the alterkation you'll have a more modern rack and pinion. you'll need an idit or flaming river steering column.

Also know that many your gauges will not work - you'll need to replace them with electrical ones. Simple stuff like the tach and speedo can be a pita.

The Gen 1 engines have forged internals and simpler ignition systems - easier to modify. However, you can have any of the ignition systems modified. And yeah, you'll need a complete efi setup - custom/modified tank, pumps, etc.

The radiator will need to be custom made - first its large, second it has bigger 2" inlets/outlets. I ended up not only modifying the firewall/trans tunnel but also moved the radiator in front of the radiator support. This means customizing the hood latch, A/C, etc. etc.

Its certainly far from a simple swap - no one makes swap components. Especially exhaust.

Good luck