What year 261?
Will you end up 6v or 12v

If it is a 54-55-1st, not many modifications would be needed.

1955-2nd through 1963 261 would most likely require (at least):
- a modification of the front motor mount.
- use your 216 clutch housing and clutch assembly.
- 6v or 12v?
- 6v or 12v generator/alternator?
- you would most likely want use a water pump relocation plate on the 261.
- an adapter for the oil pressure sender would be needed.

All of the above have been discussed regularly in the past. Spend a little time reading past posts ( and maybe try the "search" feature, which takes a little getting used to).

Maybe others will make additions and corrections to the list - and maybe people will provide links to previous discussions (I'll do that later, when I get home - tomorrow, maybe).