Everybody thinks that but, yes, they are safe. I have an Eberspacher BN2. It puts out up to 18,000 BTU which is more than enough to make a 356/550 toasty. It offered one heat output level and they typically go for upwards of $100 on eBay but you must know a bit about them to know what you're getting. Parts are available, some from Pavel in Poland and others from Asia but the original fuel delivery system has become Unobtanium. Fortunately, I have a modern solution to that which works excellently with new parts readily available (new fuel pump and associated controllers). I have not seen BN2s that run on Diesel.
Danny has a more modern Webasto(?) gas heater with variable heat output and some nice bells and whistles. It is physically slightly smaller than my BN2, and puts out about the same amount of heat. Typically, they're used to heat tractor trailer sleeper cabs so heating a 356 is easy. They run on gasoline or diesel. I think the real (not Asian knock-off) versions go for around $1,200 complete.
I tucked mine down in the nose area next to the battery. It pulls fresh air in from just ahead of the wheel well:
