early a 5 speed converson

Well, it isn't as easy as bolting it on. There are other modifications.

You need to get the OD bellhousing, but you need to bring the bellhousing and the input shaft retainer to a machininst so they can turn down the outer diameter of the retainer to match the bellhousing.

You'll probably also need to modify your driveline. Since your car is a 1961, then you most likely have the ball and trunnion style connection to the driveline. It will need to re-lengthened and fitted with a slip-yoke. Any driveline shop can do this. I spent $100 for this, and that price seems typical.

Also, you'll need to hunt down shift rods that connect your shifter to the transmission. I now forget which, but when I swapped in an A883OD I had to replace two of them. I think only the reverse rod was kept. The new rods have a different orientation and 3rd/4th gear is flipped, so at least that rod needs to be replaced.

While you're thinking about this, the OD bellhousing is probably going to be a 10.5" housing because most OD transmissions arrived after 1975 for cars (avoid the truck 833). Can your 1961 accommodate a 10.5" bellhousing?

If so, then you also need a different shift fork and pivot. Brewer's Performance can sell you what you need.

Good luck!