Get a battery post kill switch - I have gone as long as 4 months and still had a good battery.

Be aware that many 544s lose fuel during vacations from running. Put some fuel in the carb throats when you try to start the car after one of these vacations. Don't forget that under these conditions you may not be able to prevent the car from running fast - be ready to shut off the car if it revs high.

I have drilled small holes in my air cleaners, right over the carb throats. I turn the engine over a bit to get some oil in it and to see if it will start, then I prime it with an eye dropper. Two small squirts in each carb and the car starts immediately!

After the car can be idled down a bit, I seal the holes with short sheet metal screws that have a flat washer and a rubber washer.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '12 XC70, the first three are mine, heh, heh, 525,000 miles put on 10 bricks

I am a rolling advertisement for the Sears Die Hard Gold Battery.

Group Size 34 - Model Number 33334

10 1/4" x 6 3/4" and 7" Tall makes it a perfect fit for the 544 battery shelf and hold down bracket.

795 CCA and the terminals are located correctly for the Firewall mounted ground strap and Drivers side Positive terminal.

In checking with the Sear's web site I find that the current "List Price" is $129.99 with exchange. 3 year warranty.

However.... I purchased mine "used" in 2007 and it has performed flawlessly ever since. Advertised as maintenance free and that it is. Holds a charge over extended periods (I use a "float charger" to maintain the battery during longer periods of idleness - which in my case are infrequent and normally Hot Weather related) Bonus? It looks like a 1960's vintage 12v battery

According to my 544 owners manual (published 10/64), the stock battery was a "Boliden 107 GM60". Some quick research online indicates the GM60 probably refers to BCI Group 60, which measures (approximately) 13L x 6.5W x 9H. As George suggested though, I would measure the battery box in the car to see if this makes sense. On that subject, I have a Road & Track road test photo that shows the battery installation in an as-delivered '63 544, and it appears the stock battery filled the battery box nicely... not much space left over in either direction.

Also, in the photo, it appears the battery is configured as you surmised: as installed, the negative (ground) post is in the aft right (passenger) corner of the battery, while the positive post is in the left forward corner. I assume this coincides with the standard Group 60 layout, but again - you might want to check that out before you buy.
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Gary L - 142E ITB race car, 73 1800ES
BlueBrick Racing Website
YouTube Racing Videos

As for a vintage look, I have been considering painting my Interstate battery black!
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '12 XC70, the first three are mine, heh, heh, 525,000 miles put on 10 bricks

I got rid of my last PV in 1970 and it was 6volts so I am not much help.
But you could measure the space and see what battery group fits there.
If a 24 will fit, it would work well.
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George Downs Bartlesville, Oklahoma