.100 over slugs (NOT Weiscos)

75-80 Toyota 20R piston is an 88.40mm, thats a 3.4803". Check compressin height and pin diameter. Maybe a possibility as the 20R is a very popular motor and I bet there is some HP parts available...http://www.ebay.com/itm/75-80-Toyota-Celica-Corona-2-0L-20R-Piston-Ring-Set-/260789452121

here is a silvolite L6 piston to compare:
S6 225
3.7 L
3.400(IN)
86.4(MM)

CHRYSLER
ENGINE

1960-87

8.2

Cars through 1983, trucks through 1987. Flat top.

Comp. Ht. 1.740
Pin Dia..9012 Offset

Here is Toyota

L4-133.6
2189cc
3.484(IN)
88.5(MM) +0.1mm

TOYOTA
ENGINE

1975-80

8.4

20R engine. .110" crown.

Comp. Ht. 1.510
Pin Dia..8665 Offset

So they sit lower in the bore (bad unless you got tall rods) and the piston pin looks close enough to bore out to a L6 size (+.0347)

good reference page for available piston sizes http://webpages.charter.net/beckracing/silvtoc.htm