I'd be inclined to have your existing bellhousing modified.
I am currently putting a 253 into a Nissan ute, with a 6cyl Falcon T5 behind it
I will be modifying the 253 bellhousing - shortening and changing bolt pattern for gearbox. As many gearboxes are roughly the same length you should find your old tailshaft will fit - provided the yoke fits the box!
In any case it's about $150 to have a tailshaft modified and roughly the same to do the bellhousing.
Proud TQE Operator and President of SEFAG - the Society for the Elimination of Ford Automatic Gearboxes
Pacemaker headers + 2.5" exhaust
EF Fairmont interior
Lowered King springs
5 speed manual
Newest mod: EF Ghia wiring loom and dash