Cheapest solution is to rebuild/replace the existing 258, either DIY or purchase a long block. Followed by:
AMC V8 swap.
After those, you lose most (if not all) of the aftermarket support for swaps, and are on your own for Hack Fab'n parts.
The Ford 4.9L/300 I6 is a great engine. However, it is a bit long.
You will need to adapt the Jeep T18 to the Ford bellhousing. IIRC, this involves drilling one hole, as the Jeep version has a slightly different bolt pattern than the Ford 'butterfly'. But here's an idea:
Sell the Jeep T18, and buy either a Ford T18 (2wd) or a Ford NP435 (2 or 4wd). Purchase the adapter from Advance or Novak, and be 100% Blue Oval from the fan back to the Tcase: