Which spacer is best?

Canton Racing has a wide variety of phenolic spacers including tapered 4-hole. I didn't make any comparisons, but the 1" phenolic tapered 4-hole was what I used on my 360 Dart Sport. Better insulating qualities than aluminum with the tapered bores, or whichever style you choose.

As with most other engine modifications, any changes you make may benefit from re-jetting the carb for optimum results.