Re: bearing spacers?
Yes they do make them. However there is only one brand that is worth using. King bearing spacers are made in Israel and are the only quality option for a main bearing spacer. I looked at a lot of them, all brand names including Comp Cams and they all are simply soft babbit main bearings that still retain the bearing grooves in them with and are machined to be used as spacers. King uses hard steel inserts that are machined to fit between the main saddle and bearing and have no groove in them because you don't want a groove in the spacer anymore than you'd want a groove in the block main saddle. The groove is in the bearing where it belongs.
TRW and others make a thick main bearing that is one piece and seems like it would be a good option, but I've talked to some who didn't like them for one reason or another. My brother used them without any issues on a 302 he built using a 283 crank years ago. That engine was bumping 9000 rpm on a regular basis with 12.5:! comp, 4.88 gears, tunnel ram and twin predator carbs. The blocks always gave out first for him.
With the spacer make sure it's absolutely clean when installing it. I had concerns about using them myself, but on my destroked 400 I'm using Bowtie rods which are fairly heavy and the bob weight is such that even an aftermarket crankshaft like Eagle or Scat would need heavy mallory metal added to them, an expensive task to do. My current 1182 crank already had the metal in it to balance the assembly so with my lighter pistons I am using on this build I may have to remove some, but won't have to add any. I've seen the spacers listed on ebay for the small journal crank in a medium journal 350 block. Do a search for King bearing spacers. Don't waste you money on anything that Summit or Jegs is selling they are simply reworked 350 bearings, not true hardened spacers.
I agree with Mark on the rods. There are no worse rods than the 283 rods, very thin beam profile and small bolts. These are not suitable for high rpm use. Do yourself a favor and buy a good set of aftermarket connecting rods with the best bolts you can get. If you can't do that then a set of the factory small journal 327 rods from 65-67 would be the best of the small journal rods. They have more meat on the big end then the 283 rods, which don't have much. They also have a beefier beam, but the bolts are still tiny. You'll need ARP PRO-series wave loc bolts for these, not just the standard ARP wave locs. They still aren't the greatest, but they are the best of the small journal rods that GM produced.
"..............what, and get out of aviation??"